drm/amdgpu: fix error handling in amdgpu_bo_do_create

The bo structure is freed up in case of an error, so we can't do any
accounting if that happens.

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