drm/i915: Rework PPGTT init code

This rework will help if future platforms choose to be a bit different.
Should have no functional impact.

v2: Don't move around the vtable setup (Daniel)

v3: Squash in the disable-by-default patch.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>