drm/i915: Delete outdated comment in byt_pte_encode

This has been invalidated in

commit 24f3a8cf
Author: Akash Goel <akash.goel@intel.com>
Date:   Tue Jun 17 10:59:42 2014 +0530

    drm/i915: Added write-enable pte bit supportt

But despite that it's in the diff context no one noticed :(

Cc: Akash Goel <akash.goel@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>