drm/edid: Add 6 bpc quirk for SDC panel in Lenovo B50-80

Another panel that reports "DFP 1.x compliant TMDS" but it supports 6bpc
instead of 8 bpc.

Apply 6 bpc quirk for the panel to fix it.

BugLink: https://bugs.launchpad.net/bugs/1788308
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180823055332.7723-1-kai.heng.feng@canonical.com