drm/i915: Turn intel_color_check() into a vfunc

drm/i915: Turn intel_color_check() into a vfunc

The current intel_color_check() is a mess, and worse yet it is
in fact incorrect for several platforms. The hardware has
evolved quite a bit over the years, so let's just go for a clean
split between the platforms by turning this into a vfunc.
The actual work to split it up will follow.

v2: Assign the vfuncs in the order they appear in the
    struct (Matt)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-3-ville.syrjala@linux.intel.com