drm/i915: Cleaning up the relocate entry function

As the relocate entry function was getting a bit too big I've moved
the code that used to use either the cpu or the gtt to for the
relocation into two separate functions.

Signed-off-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>