backend/drm: improve logs in drm_fb_create

Downgrade errors to DEBUG level, because drm_fb_create is used in
test_buffer, so errors aren't always fatal. Add ERROR logs at call
sites where a failure is fatal, to make it clear something wrong
happened.

backend/drm/drm.c

@@ -474,6 +474,7 @@ static bool drm_connector_commit_buffer(struct wlr_output *output) {
 		}
 		if (!drm_fb_import(&plane->pending_fb, drm, buffer,
 				&crtc->primary->formats)) {
+			wlr_log(WLR_ERROR, "Failed to import buffer");
 			return false;
 		}
 		break;