drm:amd:amdgpu: Fix missing bo unlock in failure path

smatch warning - inconsistent handling of buffer object reserve
and unreserve.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>