drm/radeon/ttm: don't store driver copy of device pointer.

This can be gotten back from bdev.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826014428.828392-3-airlied@gmail.com