drm/i915/dp: fall back to 18 bpp when sink capability is unknown

drm/i915/dp: fall back to 18 bpp when sink capability is unknown

Per DP spec, the source device should fall back to 18 bpp, VESA range
RGB when the sink capability is unknown. Fix the color depth
clamping. 18 bpp color depth should ensure full color range in automatic
mode.

The clamping has been HDMI specific since its introduction in

commit 996a2239f93b03c5972923f04b097f65565c5bed
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Apr 19 11:24:34 2013 +0200

    drm/i915: Disable high-bpc on pre-1.4 EDID screens

Cc: stable@vger.kernel.org
Reported-and-tested-by: Dihan Wickremasuriya <nayomal@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=105331
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452695720-7076-1-git-send-email-jani.nikula@intel.com