drm/i915: Move the remaining gtt code

It's pretty much all consolidated now that we've killed AGP. We can move
the one outlier, and defines too.

(Kill some unused defines in the process)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>