drm/i915: split LVDS update code out of i9xx_crtc_mode_set

Just to make things clearer and reduce the size of this monstrosity.

v2: make sure 8xx PLL update function calls update_lvds too (Daniel)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
danvet: fixed patch ordering to avoid breaking bisect.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>