[Intel-gfx] PCH eDP fixes

[Jesse Barnes]

Here's the set of PCH eDP fixes I came up with once I received my Sony
Vaio.  I found a few, non-PCH issues in the process, and took the
opportunity to enhance our eDP support to avoid most of the DP training
if the VBIOS gives us good data.

This patchset is against ickle's drm-intel-next branch as of a few hours
ago, and is working on my test machine.  Before I rebased, I had success
reports from several users on bug
https://bugs.freedesktop.org/show_bug.cgi?id=29141.  The key change
there was to work around a newly discovered Ironlake panel power
sequencing bug (need to disable clock gating for the panel power
sequencing unit); before that change, panel bringup was very
intermittent.

Thanks,
Jesse