drm/amdgpu/userq: unmap queues amdgpu_userq_mgr_fini()

This was missed when the map and unmap were split out
of the mqd create and destroy functions.

Fixes: b0db33c8 ("drm/amdgpu/userq: rework front end call sequence")
Reviewed-by: Prike Liang <Prike.Liang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>