compositor-drm: use DRM constants

The GBM and DRM constants have the same meaning. In preparation
to make the DRM backend compile without libgbm, prefer the DRM
constants where GBM is not needed.

Signed-off-by: Stefan Agner <stefan@agner.ch>

libweston/backend-drm/drm.c

@@ -901,7 +901,7 @@ drm_output_find_special_plane(struct drm_backend *b, struct drm_output *output,
 
 		switch (type) {
 		case WDRM_PLANE_TYPE_CURSOR:
-			format = GBM_FORMAT_ARGB8888;
+			format = DRM_FORMAT_ARGB8888;
 			break;
 		case WDRM_PLANE_TYPE_PRIMARY:
 			/* We don't know what formats the primary plane supports
@@ -1425,10 +1425,10 @@ drm_output_init_pixman(struct drm_output *output, struct drm_backend *b)
 	uint32_t flags = 0;
 
 	switch (format) {
-		case GBM_FORMAT_XRGB8888:
+		case DRM_FORMAT_XRGB8888:
 			pixman_format = PIXMAN_x8r8g8b8;
 			break;
-		case GBM_FORMAT_RGB565:
+		case DRM_FORMAT_RGB565:
 			pixman_format = PIXMAN_r5g6b5;
 			break;
 		default:
@@ -3231,7 +3231,7 @@ drm_backend_create(struct weston_compositor *compositor,
 
 	compositor->backend = &b->base;
 
-	if (parse_gbm_format(config->gbm_format, GBM_FORMAT_XRGB8888, &b->gbm_format) < 0)
+	if (parse_gbm_format(config->gbm_format, DRM_FORMAT_XRGB8888, &b->gbm_format) < 0)
 		goto err_compositor;
 
 	/* Check if we run drm-backend using weston-launch */