graphics/drm-66-kmod: Fix conflict

WHile here add the conflict in all drm kmod ports for drm-66-kmod.

Reported by: Alonso Cárdenas Márquez <acm@FreeBSD.org>
Fixes: 00508c01e276 ("graphics/drm-66-kmod: Add new port")
Sponsored by: Beckhoff Automation GmbH & Co. KG