drm/i915: Add limited color range readout for HDMI/DP ports on g4x/vlv/chv

The limited color range knob is in the port registers on
g4x and vlv/chv for HDMI, and on g4x for DP. Add the relevant code
to read out the hardware state into pipe config. On vlv/chv the
DP port limited color range knob is in PIPECONF for which we
already have readout code.

Cc: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>