drm/i915: properly restore the ppgtt page directory on resume

The ppgtt page directory lives in a snatched part of the gtt pte
range. Which naturally gets cleared on hibernate when we pull the
power. Suspend to ram (which is what I've tested) works because
despite the fact that this is a mmio region, it is actually back by
system ram.

Fix this by moving the page directory setup code to the ppgtt init
code (which gets called on resume).

This fixes hibernate on my ivb and snb.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>