drm: fix typo in edid vendor parsing.

Should be,

    edid_vendor[2] = (edid->mfg_id[1] & 0x1f) +  '@';

Since vendor ID has only two bytes only, I am somewhat surprised why gcc
doesn't complain this.

Reported-by: Guo, Chaohong <chaohong.guo@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>