drm/i915: clarify reasoning for the access_ok call

This clarifies the comment above the access_ok check so a missing
VERIFY_READ doesn't alarm anyone.

v2:
 - rewrote comment, thanks to Chris Wilson

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: add patch history log to commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>