drm/ttm: init mem->bus in common code.

The drivers all do the same thing here.

Reviewed-by: Christian König <christian.koenig@amd.com> for both.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200811074658.58309-1-airlied@gmail.com