drm/edid: abstract OUI conversion to 24-bit int

Replace the open coded OUI conversion from three bytes to a 24-bit int,
as we'll be adding one more user shortly. No functional changes.

Side note: CTA-861 format has the OUI bytes in reverse order.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2f43032d5f001510c7eed059321ceeb76d07a606.1630419362.git.jani.nikula@intel.com