drm/radeon/ttm: move to driver binding/destroy functions. (v2)

Do agp decision in the driver, instead of special binding funcs

v2: use container_of, drop some {}.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907204630.1406528-6-airlied@gmail.com