dri2: reply to client for WaitMSC request in any case

otherwise client would wait for reply forever and desktop appears hang.

Signed-off-by: Flora Cui <flora.cui@amd.com>
(Ported from amdgpu commit fb06fb814700a47464abd756e1111dcc76d0d776)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

src/radeon_dri2.c

@@ -1156,6 +1156,9 @@ static int radeon_dri2_schedule_wait_msc(ClientPtr client, DrawablePtr draw,
 out_complete:
     if (wait_info)
 	radeon_dri2_deferred_event(NULL, 0, wait_info);
+    else
+	DRI2WaitMSCComplete(client, draw, 0, 0, 0);
+
     return TRUE;
 }