agp/intel: Fix resume regression from 2d2430cf

On i915 [EeePCs] something scribles over the registers during suspend
and resume so we must save a copy of the PGETBL_CTL register programmed
by the BIOS and restore that upon resume.

Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>