Some eDP fixes/improvements.

[Mario Kleiner]

Hi and happy new year!

Since i now have a MBP 2017 to play with, with a 10 bit Retina panel,
and Polaris gpu, i'm trying to get it to get 10 bits, and found one
small bug [fix: patch1], and a quirk of Apples Retina eDP sink, for
which i propose patch2 as solution. I sent a similar patch to i915 to
make 10 bit Retina work with the Intel Kabylake igp on that machine.

If these make sense, it would be cool to still get them into drm-fixes
for Linux 5.5, so users of spring distro updates like Ubuntu 20.04 LTS
can get a more colorful new year.

thanks,
-mario