[Intel-gfx] Splitting HAS_PCH_SPLIT code in intel_crtc_mode_set().

[Eric Anholt]

In doing new chipset enablement for the first time in quite a while,
it sunk in just how bad the spaghetti in our modeset path was these
days.  I think that splitting the crtc_mode_set up a bit gets us a
more readable path for whichever generation you're trying to look at,
while not duplicating too much code -- note that patch 1/9 is +675
LOC, while after 8/9 we're down to +299 overall.

To atone for about +300LOC, patch 9/9 does another cleanup I think is
appropriate and cuts almost 300 lines of used-once #define spam.

This is only tested on my Ironlake so far, and only for LVDS.