drm/i915: Disable 12bpc hdmi for now

It's totally broken, and since

commit d328c9d7
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Apr 10 16:22:37 2015 +0200

    drm/i915: Select starting pipe bpp irrespective or the primary plane

the kernel will try to use it even for the common rgb888 framebuffers.
Ville has patches to fix it all up properly, but unfortunately they're
stuck in review limbo. And since the 4.2 feature cutoff has passed we
need to somehow  handle this regression.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>