intel-gtt: save PGETBL_CTL later in the setup process

... and switch to a more classical store-reg-on-suspend, restore-on-resume
way of doing things. Obviously this is just preparation for the future,
the code is not there at all, yet.

This is needed because the next patch adjusts this register and everything
in it (not just the pagetable address) needs to be restored on resume.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>