drm/amdgpu: make sure we unpin the UVD BO

Releasing pinned BOs is illegal now. UVD 6 was missing from:
commit 2f40801d ("drm/amdgpu: make sure we unpin the UVD BO")

Fixes: 2f40801d

 ("drm/amdgpu: make sure we unpin the UVD BO")
Cc: stable@vger.kernel.org
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>