drm/i915: add yesno utility function

Add a common function to return "yes" or "no" string based on the
argument, and drop the local versions of it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>