drm/amd/display: Fail atomic_check early on normalize_zpos error

[Why]

drm_atomic_normalize_zpos() can return an error code when there's
modeset lock contention. This was being ignored.

[How]

Bail out of atomic check if normalize_zpos() returns an error.

Fixes: b261509952bc ("drm/amd/display: Fix double cursor on non-video RGB MPO")
Signed-off-by: Leo Li <[e-mail]>
Tested-by: Mikhail Gavrilov <[e-mail]>
Reviewed-by: Hamza Mahfooz <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 	 * `dcn10_can_pipe_disable_cursor`). By now, all modified planes are in
 	 * atomic state, so call drm helper to normalize zpos.
 	 */
-	drm_atomic_normalize_zpos(dev, state);
+	ret = drm_atomic_normalize_zpos(dev, state);
+	if (ret) {
+		drm_dbg(dev, "drm_atomic_normalize_zpos() failed\n");
+		goto fail;
+	}
 
 	/* Remove exiting planes if they are modified */
 	for_each_oldnew_plane_in_state_reverse(state, plane, old_plane_state, new_plane_state, i) {