Revert "drm/amdgpu/gmc11: avoid cpu accessing registers to flush VM"

This reverts commit 8748de87
since drv enabled mes to access registers.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>