drm/edid: make a number of functions, parameters and variables const

If there's no need to change it, it should be const. There's more to be
done, but start off with changes that make follow-up work easier. No
functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/41722f92ef81cd6adf65f936fcc5301418e1f94b.1617024940.git.jani.nikula@intel.com