[Intel-gfx] Share common logic between eDP and LVDS in panel

[Chris Wilson]

The eDP panel handling lacked a lot of the finese that we have developed
for LVDS, such as handling lid notifications and cacheing EDIDs. Also we
had a lot of connector specific data stuffed into the global
drm_i915_private which makes for a confusing read.

Let's try to reduce that confusion by sharing the common logic.
-Chris