drm/i915: Replace some PAGE_SIZE with I915_GTT_PAGE_SIZE

Use I915_GTT_PAGE_SIZE when talking about GTT pages rather than
physical pages.

There are some PAGE_SHIFTs left though. Not sure if we want to
introduce I915_GTT_PAGE_SHIFT or what?

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk> # at least some of it :)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180913150405.706-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>