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: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5a11435419d804a58ef356c0b9acf445cffc1354.1648752228.git.jani.nikula@intel.com

drivers/gpu/drm/drm_edid.c

@@ -1842,8 +1842,8 @@ static struct edid *edid_filter_invalid_blocks(const struct edid *edid,
 			memcpy(dest_block++, block, EDID_LENGTH);
 	}
 
-	new->checksum += new->extensions - valid_extensions;
 	new->extensions = valid_extensions;
+	new->checksum = edid_block_compute_checksum(new);
 
 out:
 	kfree(edid);