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

Due to gfxoff on, cpu accessing registers is not expected.

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>