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/drm.h

@@ -26,6 +26,8 @@ struct wlr_drm_plane {
 	struct wlr_drm_surface surf;
 	struct wlr_drm_surface mgpu_surf;
 
+	uint32_t drm_format; // ARGB8888 or XRGB8888
+
 	// Only used by cursor
 	float matrix[9];
 	struct gbm_bo *cursor_bo;