backend/drm: fix GBM format mismatch

We create the EGL config with GBM_FORMAT_ARGB8888, but then initialize GBM BOs
with GBM_FORMAT_XRGB8888. This mismatch confuses Mesa.

Instead, we can always use GBM_FORMAT_ARGB8888, and use DRM_FORMAT_XRGB8888
when calling drmModeAddFB2.

Fixes https://github.com/swaywm/wlroots/issues/1438

include/backend/drm/renderer.h

@@ -16,6 +16,8 @@ struct wlr_drm_renderer {
 	struct gbm_device *gbm;
 	struct wlr_egl egl;
 
+	uint32_t gbm_format;
+
 	struct wlr_renderer *wlr_rend;
 };