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>