drm/i915: Remove todo and comments about struct_mutex

This patch completes the removal of struct_mutex from the driver.

Remove the related TODO item, as the transition away from struct_mutex
is now complete.

Also clean up references to struct_mutex in i915.rst to avoid outdated
documentation.

Signed-off-by: Luiz Otavio Mello <luiz.mello@estudante.ufscar.br>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250908131518.36625-10-luiz.mello@estudante.ufscar.br
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Documentation/gpu/i915.rst

@@ -358,8 +358,6 @@ Locking Guidelines
 #. All locking rules and interface contracts with cross-driver interfaces
    (dma-buf, dma_fence) need to be followed.
 
-#. No struct_mutex anywhere in the code
-
 #. dma_resv will be the outermost lock (when needed) and ww_acquire_ctx
    is to be hoisted at highest level and passed down within i915_gem_ctx
    in the call chain
@@ -367,11 +365,6 @@ Locking Guidelines
 #. While holding lru/memory manager (buddy, drm_mm, whatever) locks
    system memory allocations are not allowed
 
-	* Enforce this by priming lockdep (with fs_reclaim). If we
-	  allocate memory while holding these looks we get a rehash
-	  of the shrinker vs. struct_mutex saga, and that would be
-	  real bad.
-
 #. Do not nest different lru/memory manager locks within each other.
    Take them in turn to update memory allocations, relying on the object’s
    dma_resv ww_mutex to serialize against other operations.

Documentation/gpu/todo.rst

@@ -173,31 +173,6 @@ Contact: Simona Vetter
 
 Level: Intermediate
 
-Get rid of dev->struct_mutex from GEM drivers
----------------------------------------------
-
-``dev->struct_mutex`` is the Big DRM Lock from legacy days and infested
-everything. Nowadays in modern drivers the only bit where it's mandatory is
-serializing GEM buffer object destruction. Which unfortunately means drivers
-have to keep track of that lock and either call ``unreference`` or
-``unreference_locked`` depending upon context.
-
-Core GEM doesn't have a need for ``struct_mutex`` any more since kernel 4.8,
-and there's a GEM object ``free`` callback for any drivers which are
-entirely ``struct_mutex`` free.
-
-For drivers that need ``struct_mutex`` it should be replaced with a driver-
-private lock. The tricky part is the BO free functions, since those can't
-reliably take that lock any more. Instead state needs to be protected with
-suitable subordinate locks or some cleanup work pushed to a worker thread. For
-performance-critical drivers it might also be better to go with a more
-fine-grained per-buffer object and per-context lockings scheme. Currently only
-the ``msm`` and `i915` drivers use ``struct_mutex``.
-
-Contact: Simona Vetter, respective driver maintainers
-
-Level: Advanced
-
 Move Buffer Object Locking to dma_resv_lock()
 ---------------------------------------------