Fix GPU triggering random system read after VRAM start change

[j.glisse at gmail.com]

So first patch is a fix in itself, smallest possible and should go to
stable. Second patch is an improvement as a first step to flicker free
boot.

I have yet extensively tested second patch, especialy not on AGP but
so far on few GPU/motherboard it looks good. It can probably wait 3.7.
Will test it more and report.

I have a third patch that is a step closer to flicker free boot on uefi,
waiting ack to release a reg.

Cheers,
Jerome