drm/i915: Extract PPGTT pte encoding

HSW will change the PTE encoding, and laying this out now will be
helpful when we're ready to implement that. More importantly, GGTT and
PPGTT PTE encoding is quite similar, so moving this out into a helper
function will enable us to lance the AGP layer.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>