Just like we did for pipe gamma, let's also track the pipe csc
state. The hardware only exists on ILK+, and currently we always
enable it on hsw+ and never on any other platforms. Just like
with pipe gamma, the primary plane control register is used
for the readout on pre-SKL, and the pipe bottom color register
on SKL+.

v2: Rebase
v3: Allow fastboot with csc_enable changes (Maarten)
    Deal with HAS_GMCH
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207202146.26423-4-ville.syrjala@linux.intel.com