drm/i915: Introduce enableddisabled helper

Similar to existing yesno and onoff and use it throughout the code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479385814-2358-2-git-send-email-tvrtko.ursulin@linux.intel.com