drm: make drm_get_format_name thread-safe

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
[danvet: Clarify that the returned pointer must be freed with
kfree().]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index fa39307..087391f 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -837,8 +837,9 @@
 	/* Check whether this plane supports the fb pixel format. */
 	ret = drm_plane_check_pixel_format(plane, state->fb->pixel_format);
 	if (ret) {
-		DRM_DEBUG_ATOMIC("Invalid pixel format %s\n",
-				 drm_get_format_name(state->fb->pixel_format));
+		const char *format_name = drm_get_format_name(state->fb->pixel_format);
+		DRM_DEBUG_ATOMIC("Invalid pixel format %s\n", format_name);
+		kfree(format_name);
 		return ret;
 	}