The function will also be used by a later patch, so factor it out.

V2: make raw_edid const, define/declare before first use
V3: fix erroneuos removal of csum variable
Signed-off-by: NStefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>