drm/i915: properly set ppgtt cacheability on snb

For some reason snb has 2 fields to set ppgtt cacheability. This one
here does not exist on gen7.

This might explain why ppgtt wasn't a win on snb like on ivb - not
enough pte caching.

v2: Fixup rebase fail.

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