drm/i915: rename PPGTT/GGTT fields OA registers

We had a generic field name used across 2 registers but it feels like
it's clearer we make it obvious what register this field belongs to.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326090831.22686-7-lionel.g.landwerlin@intel.com