drm/amd/display: Allow cursor async updates for framebuffer swaps

[Why]
We previously allowed framebuffer swaps as async updates for cursor
planes but had to disable them due to a bug in DRM with async update
handling and incorrect ref counting. The check to block framebuffer
swaps has been added to DRM for a while now, so this check is redundant.

The real fix that allows this to properly in DRM has also finally been
merged and is getting backported into stable branches, so dropping
this now seems to be the right time to do so.

[How]
Drop the redundant check for old_fb != new_fb.

With the proper fix in DRM, this should also fix some cursor stuttering
issues with xf86-video-amdgpu since it double buffers the cursor.

IGT tests that swap framebuffers (-varying-size for example) should
also pass again.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: David Francis <david.francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4544,20 +4544,10 @@ static int dm_plane_atomic_check(struct drm_plane *plane,
 static int dm_plane_atomic_async_check(struct drm_plane *plane,
 				       struct drm_plane_state *new_plane_state)
 {
-	struct drm_plane_state *old_plane_state =
-		drm_atomic_get_old_plane_state(new_plane_state->state, plane);
-
 	/* Only support async updates on cursor planes. */
 	if (plane->type != DRM_PLANE_TYPE_CURSOR)
 		return -EINVAL;
 
-	/*
-	 * DRM calls prepare_fb and cleanup_fb on new_plane_state for
-	 * async commits so don't allow fb changes.
-	 */
-	if (old_plane_state->fb != new_plane_state->fb)
-		return -EINVAL;
-
 	return 0;
 }