drm/i915/gtt: Unmap the PCI pages after unbinding them from the GTT

Dave Airlie spotted that his ILK laptop with DMAR enabled was generating
the occasional DMAR warning.

"The ordering in the previous code was to rewrite the GTT table before
unmapping the pages and that makes sense to me."

This is his stable patch ported to d-i-n.

Reported-by: Dave Airlie <airlied@redhat.com>
Original-patch-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>