backend-drm: use pixel format to print gbm format

The gbm_format is the same as the drm format used by the pixel format.

Print the format name using the pixel format in the error message to make the
error message easier to understand for humans.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>

libweston/backend-drm/drm-gbm.c

@@ -183,6 +183,7 @@ create_gbm_surface(struct gbm_device *gbm, struct drm_output *output)
 {
 	struct weston_mode *mode = output->base.current_mode;
 	struct drm_plane *plane = output->scanout_plane;
+	const struct pixel_format_info *pixel_format;
 	struct weston_drm_format *fmt;
 	const uint64_t *modifiers;
 	unsigned int num_modifiers;
@@ -190,8 +191,15 @@ create_gbm_surface(struct gbm_device *gbm, struct drm_output *output)
 	fmt = weston_drm_format_array_find_format(&plane->formats,
 						  output->gbm_format);
 	if (!fmt) {
-		weston_log("format 0x%x not supported by output %s\n",
-			   output->gbm_format, output->base.name);
+		pixel_format = pixel_format_get_info(output->gbm_format);
+		if (pixel_format)
+			weston_log("format %s not supported by output %s\n",
+				   pixel_format->drm_format_name,
+				   output->base.name);
+		else
+			weston_log("format 0x%x not supported by output %s\n",
+				   output->gbm_format,
+				   output->base.name);
 		return;
 	}