Make the code selecting the RGB quantization range a little less magicy
by wrapping it up in a small helper.

v2: s/adjusted_mode/mode in vc4 to make it actually compile
v3: Add a comment proposed by Eric
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170111141835.25369-1-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Eric Anholt <eric@anholt.net>