drm/amdgpu: Remove explicit wait after VM validate

PD or PT might have to be moved during validation and this move has to be
completed before updating it. If page table updates are done using SDMA
then this serializing is done by SDMA command submission.

And if PD/PT updates are done by CPU, then explicit waiting for PD/PT
updates are done in amdgpu VM amdgpu_vm_wait_pd function.

Sync to PD BO moving fence to handle corner case where none of the PTs
are updated but PD is evicted.

Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>