drm/i915: Split the crtc_mode_set function along HAS_PCH_SPLIT() lines.

This path, which shouldn't be *that* complicated, is now so littered
with per-chipset tweaks that it's hard to trace the order of what
happens.  HAS_PCH_SPLIT() is the most radical change across chipsets,
so it seems like a natural split to simplify the code.

This first commit just copies the existing code without changing
anything.

v2: updated to track removal of call to intel_enable_plane from i9xx_crtc_mode_set

Signed-off-by: Eric Anholt <eric@anholt.net>
Hella-acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>