backend/drm: use EGL_PLATFORM_GBM_KHR

This is just the vendor-agnostic define for the GBM platform. It has the
same value as EGL_PLATFORM_GBM_MESA, so should work with old drivers
that only offer the Mesa-vendored extension too.

backend/drm/renderer.c

@@ -42,8 +42,8 @@ bool init_drm_renderer(struct wlr_drm_backend *drm,
 
 	renderer->gbm_format = GBM_FORMAT_ARGB8888;
 	renderer->wlr_rend = create_renderer_func(&renderer->egl,
-		EGL_PLATFORM_GBM_MESA, renderer->gbm,
-		config_attribs, renderer->gbm_format);
+		EGL_PLATFORM_GBM_KHR, renderer->gbm, config_attribs,
+		renderer->gbm_format);
 	if (!renderer->wlr_rend) {
 		wlr_log(WLR_ERROR, "Failed to create EGL/WLR renderer");
 		goto error_gbm;