drm/i915/bdw: remove preliminary_hw_support flag from BDW

It ought to work ok in 3.14.  We have some fun stuff coming after that,
but all the basics are in place now and seem relatively stable.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>