drm/i915: state readout and cross checking for limited_color_range

At least on those platforms which have a simple bit and don't rely
on the fully programmable CSC unit to do this.

Note that with the current code this includes CHV, but I guess that
platform will match BYT.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>