drm/amdgpu/ttm: handle tt moves properly.

The core move code currently handles use_tt moves, for amdgpu
this was being handled also in the driver, but not using the same
paths.

If moving between TT/SYSTEM (all the use_tt paths on amdgpu) use
the core move function.

Eventually the core will be flipped over to calling the driver.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924051845.397177-4-airlied@gmail.com