drm/i915: remove intel_gtt structure

With the probe call in our dispatch table, we can now cut away the
last three remaining members in the intel_gtt shared struct and so
remove it completely.

v2: Rebased on top of Daniel's series

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: bikeshed commit message a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>