drm/i915: Write PDEs at init instead of enable

We won't be calling enable() for all PPGTTs. We do need to write PDEs
for all PPGTTs however. By moving the writing to init (which is called
for all PPGTTs) we should accomplish this.

ADD NOTE ABOUT PDE restore

TODO: Eventually, we should allocate the page tables on demand.

v2: Rebased on BDW. Only do PDEs for pre-gen8

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