compositor-drm: Remove unused output reference in drm_fb

Signed-off-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor-drm.c

@@ -139,10 +139,7 @@ struct drm_mode {
 	drmModeModeInfo mode_info;
 };
 
-struct drm_output;
-
 struct drm_fb {
-	struct drm_output *output;
 	uint32_t fb_id, stride, handle, size;
 	int fd;
 	int is_client_buffer;