drm/edid: Extract drm_mode_cea_vic()

Extract the logic to compute the final CEA VIC to a small helper.
We'll reorder it a bit to make future modifications more
straightforward. No function changes.

Cc: Wayne Lin <waynelin@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004141914.20600-2-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>