drm/i915/overlay: Replace i915_gem_obj_ggtt_offset() with the known flip_addr

When setting up the overlay page, we pin it into the GGTT (when using
virtual addresses) and store the offset as overlay->flip_addr. Rather
than doing a lookup of the GGTT address everytime, we can use the known
address instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-2-git-send-email-chris@chris-wilson.co.uk