EDID/DP color precision fixes on Intel hw for stable

[Mario Kleiner]

Bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=105331

received a potential fix that was backported to stable. While that
patch itself is correct for treating DP video sinks with "unknown
color depth", it uncovered some lack in our general EDID 1.3
handling, and in how we treat DP->DVI/VGA, causing the fall back
of Intel DP to 6 bpc / 18 bpp in cases where it shouldn't fall
back. That leads to unhappy neuroscience/medical users of Intel gpus
which need their DP->DVI or DP->VGA display devices to operate at at
least 8 bpc without dithering.

The following three patches try to improve our EDID handling and
Intel DP to try harder to detect the proper bpc to avoid these
regressions for DP-DVI and DP-VGA. The third patch tries to fix
FDO bug 105331 without causing general unhappiness of other users.

thanks,
-mario