backend/drm: save current refresh rate

wlr_output.refresh is populated by core wlr_output, and thus will
be zero for a custom mode with an unset refresh rate.

Save the refresh rate from the drmModeModeInfo in wlr_drm_connector
instead.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3791

backend/drm/drm.c

@@ -475,6 +475,10 @@ static bool drm_crtc_commit(struct wlr_drm_connector *conn,
 		}
 
 		drm_connector_set_pending_page_flip(conn, page_flip);
+
+		if (state->base->committed & WLR_OUTPUT_STATE_MODE) {
+			conn->refresh = calculate_refresh_rate(&state->mode);
+		}
 	} else {
 		// The set_cursor() hook is a bit special: it's not really synchronized
 		// to commit() or test(). Once set_cursor() returns true, the new
@@ -1466,6 +1470,7 @@ static bool connect_drm_connector(struct wlr_drm_connector *wlr_conn,
 				wlr_conn->crtc->props.mode_id, &mode_id);
 
 			wlr_conn->crtc->mode_id = mode_id;
+			wlr_conn->refresh = calculate_refresh_rate(current_modeinfo);
 		}
 
 		wlr_log(WLR_INFO, "  %"PRId32"x%"PRId32" @ %.3f Hz %s",
@@ -1765,7 +1770,7 @@ static void handle_page_flip(int fd, unsigned seq,
 		.presented = drm->session->active,
 		.when = &present_time,
 		.seq = seq,
-		.refresh = mhz_to_nsec(conn->output.refresh),
+		.refresh = mhz_to_nsec(conn->refresh),
 		.flags = present_flags,
 	};
 	wlr_output_send_present(&conn->output, &present_event);

include/backend/drm/drm.h

@@ -175,6 +175,8 @@ struct wlr_drm_connector {
 
 	// Last committed page-flip
 	struct wlr_drm_page_flip *pending_page_flip;
+
+	int32_t refresh;
 };
 
 struct wlr_drm_backend *get_drm_backend_from_backend(