agp/intel-gtt: don't require the agp bridge on setup

We only need it to fake the agp interface and don't actually
use it in the driver anywhere. Hence conditionalize that.

This is just a prep patch to eventually disable the fake agp
driver on gen6+.

Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>