drm/amdgpu: dequeue mes scheduler during fini

[Why]
If mes is not dequeued during fini, mes will be in an uncleaned state
during reload, then mes couldn't receive some commands which leads to
reload failure.

[How]
Perform MES dequeue via MMIO after all the unmap jobs are done by mes
and before kiq fini.

v2: Move the dequeue operation inside kiq_hw_fini.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>