drm/radeon/uvd: Fix forgotten unmap buffer objects

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

Signed-off-by: zhanglianjie <zhanglianjie@uniontech.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>