drm/i915: split out pll divider code

This cleans up the mode set path a little further, making it easier to
extend for future platforms.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: shut up stupid gcc warning about potential use of
un-initlized fp2]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>