[Intel-gfx] [RFC/RFT] Reduced refresh for ILK+

[Jesse Barnes]

Here's my attempt at adding reduced refresh rate support to ILK+.  I had
to make some changes to the DP code to properly fetch the EDID when
possible, and refactor a lot of the mode setting code to accomodate
reduced refresh calculations for ILK+, obviously I didn't split things
into as many commits as I should have.

Please review & test, I need suggestions on how to split up the commits.

Thanks,
Jesse