drm/i915: enumerate scratch fields

We have a bunch of offsets in the scratch buffer. As we're about to
add some more, let's group all of the offsets in a common location.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709123351.5645-6-lionel.g.landwerlin@intel.com