Some eDP fixes/improvements.

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