drm/edid: refactor CTA Y420CMDB parsing

Now that we have pre-parsed CTA VDB VICs stored in info->vics, leverage
that to simplify CTA Y420CMDB parsing. Move updating the y420_cmdb_modes
bitmap to the display info parsing stage, instead of updating it during
add modes. This allows us to drop the intermediate y420_cmdb_map from
display info, and replace it with a local variable.

This is prerequisite work for overall better separation of the two
parsing steps (updating display info and adding modes).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7a0e5e99a83f203b6a8981d263b89b2bb7d2fe15.1672826282.git.jani.nikula@intel.com