drm/i915: split PLL update code out of i9xx_crtc_mode_set

Makes it more readable and maintainable.  ValleyView will add its own
PLL update function in a later patch.

v2: split LVDS bits out of this patch (Daniel)
v3: fix dropped DP dithering hunk (Daniel)

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
danvet:
- fixup spurious whitespace change
- reorder patches to fix bisect breakage
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>