drm/amdgpu: fix error handling in amdgpu_bo_list_get()

We should not leak the pointer where we couldn't grab the reference
on to the caller because it can be that the error handling still
tries to put the reference then.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org