drm/i915: Extract intel_hdmi_limited_color_range()

Pull the code for computing the limited color range
setting into a small helper. We'll add a bit more to it
later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-5-ville.syrjala@linux.intel.com


Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>