drm/i915: Rename the gtt_list to global_list

Since it will be used for the global bound/unbound list with full PPGTT,
this helps clarify things for upcoming code rework.

Recommended-by: Chris Wilson <chris@chris-wilson.co.uk>
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>