[Intel-gfx] [RFT] eDP setup fix

[Adam Jackson]

I got a report of KMS setup failing on an eDP panel.  dmesg shows us
trying to set 1.62GHz for the link rate, which would be perfectly legal
for the requested mode, but the VBT appears to claim that the panel
only supports 2.7GHz.

This patch attempts to scrape the eDP setup parameters out of the VBT
and use those instead of forcing something else down the panel's
throat.  I don't have any eDP machines to test with at the moment, so
wider testing would be appreciated.  In particular I'm not trying to
get the voltage or pre-emphasis levels out of the VBT either, but
hopefully that isn't necessary.

- ajax