drm/amdgpu: revert "use CPU for page table update if SDMA is unavailable"

That is clearly not something we should do upstream. The SDMA is
mandatory for the driver to work correctly.

We could do this for emulation and bringup, but in those cases the
engineer should probably enabled CPU based updates manually.

This reverts commit 62eefd10.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>