drm/edid: Don't include ext block csum in DispID size

The EDID extension block checksum byte is not part of the
actual DispID data, so don't use it in validate_displayid().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313162054.16009-8-ville.syrjala@linux.intel.com


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>