drm-backend: do not set output->crtc to NULL in drm_output_init_planes() failure

After commit "drm-backend: move code to init/deinit planes to specific
functions" we have a specific function to init planes. As this function
does not set output->crtc, it should not set it to NULL in case of
failure. This is caller's responsibility.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

libweston/backend-drm/drm.c

@@ -1664,7 +1664,6 @@ drm_output_init_planes(struct drm_output *output)
 	if (!output->scanout_plane) {
 		weston_log("Failed to find primary plane for output %s\n",
 			   output->base.name);
-		output->crtc = NULL;
 		return -1;
 	}