drm/amdgpu/ttm: move vram/gtt mgr allocations to mman.

Christian suggested this and it makes sense.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200806233459.4057784-2-airlied@gmail.com