drm/i915: Extract i9xx_set_pll_dividers

These two writes are the very last hw writes from the
->crtc_modeset_callback on pre-gen5 hardware. As usual vlv is a bit
different, so this here is just warm-up.

Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge