drm/i915: Move page sizes out of the 8-bit sandwich

Slightly smaller code and a bit more logical layout.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222111658.4999-1-tvrtko.ursulin@linux.intel.com