drm/amdgpu/userq: move waiting for last fence before umap

Need to wait for the last fence before unmapping.  This
also fixes a memory leak in amdgpu_userqueue_cleanup()
when the fence isn't signalled.

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>