drm/edid: clean up EDID block checksum functions

Have two clear functions, one to compute the checksum over the EDID, and
another to get the checksum from the EDID. Throw away the diff function.

Ditch the drm_ prefix for static functions, and accept const void * to
help transition to struct edid * usage.

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/36641401c8eb0e403c0e33365ff4ad9a28f9fd4a.1648752228.git.jani.nikula@intel.com