drm/i915: don't flood the logs about bdw semaphores

BDW is no longer flagged as preliminary hw, but without
i915.preliminary_hw_support module param set the logs are filled with
WARNs about it.

Just make semaphores off the BDW per-chip default for now.

CC: Ben Widawsky <ben@bwidawsk.net>
Reported-by: Sebastien Dufour <sebastien.dufour@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>