drm/i915: drop pointer to drm_gem_object

Luckily the change is quite a little bit less invasive than I've
feared.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>