drm/legacy: don't include any of ati_pcigart in legacy. (v2)

This could probably be done with Kconfig somehow, but I failed in my
first 2 minute attempt.

v2: use Kconfig better.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>