drm/i915: Use 200MHz cdclk on vlv when all pipes are off

Drop the cdclk frequency to 200MHz on vlv when all pipes are off. In
theory we should be able to use 200MHz also when the pixel clock is at
most 90% of 200MHz. However in practice all we seem to get is a solid
color picture or an otherwise corrupted display.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>