drm/amdgpu: execution barrier after fence v2

Insert wait for reg mem after EOP to fix potential issue with vm context switch

v2: move wait to vm_flush() use equal instead of greater than.

Signed-off-by: Anatoli Antonovitch <anatoli.antonovitch@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>