drm/nv50: DCB quirk for Dell M6300

Uncertain if this is a weirdo configuration, or a BIOS bug.  If it's not
a BIOS bug, we still don't know how to make it work anyway so ignore a
"conflicting" DCB entry to prevent a display hang.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>