drm/edid: Add 6 bpc quirk for CPT panel in Asus UX303LA

Similar to commit e10aec65 ("drm/edid: Add 6 bpc quirk for display
AEO model 0."), the EDID reports "DFP 1.x compliant TMDS" but it support
6bpc instead of 8 bpc.

Hence, use 6 bpc quirk for this panel.

Fixes: 196f954e ("drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"")
BugLink: https://bugs.launchpad.net/bugs/1749420


Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180218085359.7817-1-kai.heng.feng@canonical.com