compositor-drm: clear gbm_surface pointer after destroying the GBM surface

Since commit ee1d968e64 ("compositor-drm: Fall back if GBM surface fails with
modifiers"), drm_output_init_egl requires output->gbm_surface to be NULL, or
gbm_surface_create will not be called if HAVE_GBM_MODIFIERS is enabled but no
modifiers are supported by the plane. This could happen if _init_egl is called
after drm_ouptut_fini_egl drom drm_output_switch_mode.

Add an assert to guarantee the requirement and clears the gbm_surface pointer
after the surface is destroyed.

Signed-off-by: Philipp Zabel <p.pzabel@pengutronix.de>

libweston/compositor-drm.c

@@ -5045,6 +5045,8 @@ drm_output_init_egl(struct drm_output *output, struct drm_backend *b)
 	struct drm_plane *plane = output->scanout_plane;
 	unsigned int i;
 
+	assert(output->gbm_surface == NULL);
+
 	for (i = 0; i < plane->count_formats; i++) {
 		if (plane->formats[i].format == output->gbm_format)
 			break;
@@ -5094,6 +5096,7 @@ drm_output_init_egl(struct drm_output *output, struct drm_backend *b)
 					      n_formats) < 0) {
 		weston_log("failed to create gl renderer output state\n");
 		gbm_surface_destroy(output->gbm_surface);
+		output->gbm_surface = NULL;
 		return -1;
 	}
 
@@ -5120,6 +5123,7 @@ drm_output_fini_egl(struct drm_output *output)
 
 	gl_renderer->output_destroy(&output->base);
 	gbm_surface_destroy(output->gbm_surface);
+	output->gbm_surface = NULL;
 	drm_output_fini_cursor_egl(output);
 }