drm/amdgpu: make sure we unpin the UVD BO

Releasing pinned BOs is illegal now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210415084730.2057-1-christian.koenig@amd.com