drm/i915: Group all the global context information together

Create a substruct to hold all the global context state under
drm_i915_private.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620110547.15947-1-chris@chris-wilson.co.uk