drm/i915: Track all held rpm wakerefs

Everytime we take a wakeref, record the stack trace of where it was
taken; clearing the set if we ever drop back to no owners. For debugging
a rpm leak, we can look at all the current wakerefs and check if they
have a matching rpm_put.

v2: Use skip=0 for unwinding the stack as it appears our noinline
function doesn't appear on the stack (nor does save_stack_trace itself!)
v3: Allow rpm->debug_count to disappear between inspections and so
avoid calling krealloc(0) as that may return a ZERO_PTR not NULL! (Mika)
v4: Show who last acquire/released the runtime pm

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Tested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114142129.24398-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/Kconfig.debug

@@ -21,11 +21,11 @@ config DRM_I915_DEBUG
         select DEBUG_FS
         select PREEMPT_COUNT
         select I2C_CHARDEV
+        select STACKDEPOT
         select DRM_DP_AUX_CHARDEV
         select X86_MSR # used by igt/pm_rpm
         select DRM_VGEM # used by igt/prime_vgem (dmabuf interop checks)
         select DRM_DEBUG_MM if DRM=y
-        select STACKDEPOT if DRM=y # for DRM_DEBUG_MM
 	select DRM_DEBUG_SELFTEST
 	select SW_SYNC # signaling validation framework (igt/syncobj*)
 	select DRM_I915_SW_FENCE_DEBUG_OBJECTS
@@ -173,6 +173,7 @@ config DRM_I915_DEBUG_RUNTIME_PM
 	bool "Enable extra state checking for runtime PM"
 	depends on DRM_I915
 	default n
+	select STACKDEPOT
 	help
 	  Choose this option to turn on extra state checking for the
 	  runtime PM functionality. This may introduce overhead during