drm/amdgpu: fix BO move offsets

It's pretty pointless to get the offset first and then initialize it.

Should fix issues with the new GTT manager.

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