drm/edid: reduce magic when updating the EDID block checksum

The code modifying the EDID block should not need to do tricks to fix
the checksum. We have a function for computing the checksum, use it.

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/5a11435419d804a58ef356c0b9acf445cffc1354.1648752228.git.jani.nikula@intel.com