drm/i915: Start exploiting drm_device subclassing

Baby step, update to_i915() conversion from drm_device to
drm_i915_private:

   text	   data	    bss	    dec	    hex	filename
1108812	  23207	    416	1132435	 114793	i915.ko (before)
1104999	  23207	    416	1128622	 1138ae	i915.ko (after)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-9-git-send-email-chris@chris-wilson.co.uk