drm/i915/ppgtt: Set scratch page "globally"

The PPGTT scratch page is used for all gens, and doing it in the global
part of our PPGTT setup makes the code a bit nicer.

This was in a patch submitted earlier as part of the PPGTT cleanups.
Grumpy maintainer must have missed it, and I didn't yell when
appropriate. Apologies for everyone :-)

v2: Update commit message

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