[Intel-gfx] i9xx mode setting cleanups

[Jesse Barnes]

Just a few cleanups to the 9xx mode set paths to make them match the
ILK+ ones a bit better and make them easier to extend for future
platforms.

I'm hoping these can land in 3.3 both to get some coverage in case I
broke something and to make future patches on top of 3.3 a bit smaller
and easier to backport for OSVs using a 3.3 base.

Thanks,
Jesse