[Intel-gfx] Bug 28479 -- black screen for non-native modes on eDP

[Chris Wilson]

Yakui verified that these patches fix the issue reported in

  Bug 28479 - Black screen after using 1024x768 that manually added by xrandr.
  https://bugs.freedesktop.org/show_bug.cgi?id=28479

and then went off to enjoy his vacation.

The patches look sane in that they duplicate the panel handling similar to
the other encoder types. However, we are ending up with lots of
duplicated code and repeating mistakes. Is there any way we can begin to
refactor the fixed mode panel logic?
-Chris