drm/i915: Reduce the pointer dance of i915_is_ggtt()

The multiple levels of indirect do nothing but hinder the compiler and
the pointer chasing turns to be quite painful but painless to fix.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456484600-11477-1-git-send-email-tvrtko.ursulin@linux.intel.com