drm/amdkfd: Fix and simplify sync object handling for KFD

The adev parameter in amdgpu_sync_fence and amdgpu_sync_resv is only
needed for updating sync->last_vm_update. This breaks if different
adevs are passed to calls for the same sync object.

Always pass NULL for calls from KFD because sync objects used for
KFD don't belong to any particular device, and KFD doesn't need the
sync->last_vm_update fence.

This fixes kernel log warnings on multi-GPU systems after recent
changes in amdgpu_amdkfd_gpuvm_restore_process_bos.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>