drm/i915: Switch the global i915.semaphores check to a local predicate

Rather than use a global modparam, we can just check to see if the
engine has semaphores configured upon it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170503093924.5320-7-chris@chris-wilson.co.uk