lib/igt_edid: merge edid_ext_update_cea_checksum into edid_update_checksum

Callers no longer need to call edid_ext_update_cea_checksum on each extension
block. This makes it less error-prone to miss one block by mistake.

Signed-off-by: Simon Ser <simon.ser@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>