drm/amd/amdgpu/amdgpu_uvd: Fix forgotten unmap buffer object

After the buffer object is successfully mapped,
call amdgpu_bo_kunmap before the function returns.

Signed-off-by: zhanglianjie <zhanglianjie@uniontech.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>