drm/gem: Drop struct_mutex requirement from drm_gem_mmap_obj

Since

commit 131e663bd6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 9 23:32:33 2015 +0200

    drm/gem: rip out drm vma accounting for gem mmaps

there is no need for this any more.

v2: Fixup compile noise spotted by 0-day build.

Link: http://mid.gmane.org/1444894601-5200-9-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_gem.c

@@ -810,8 +810,6 @@ EXPORT_SYMBOL(drm_gem_vm_close);
  * drm_gem_mmap() prevents unprivileged users from mapping random objects. So
  * callers must verify access restrictions before calling this helper.
  *
- * NOTE: This function has to be protected with dev->struct_mutex
- *
  * Return 0 or success or -EINVAL if the object size is smaller than the VMA
  * size, or if no gem_vm_ops are provided.
  */
@@ -820,8 +818,6 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
 {
 	struct drm_device *dev = obj->dev;
 
-	lockdep_assert_held(&dev->struct_mutex);
-
 	/* Check for valid size. */
 	if (obj_size < vma->vm_end - vma->vm_start)
 		return -EINVAL;