drm/i915: Trivial grammar fix s/opt of/opt out of/ in comment

The word out was dropped from the sentence across the line break, put it
back.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170816085210.4199-6-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>