drm/i915: embed the gem object into drm_i915_gem_object

Just embed it and adjust the pointers, No other changes (that's
for later patches).

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