Instead of defining the actualy user and group id in the drmP.h files
define GID_VIDEO in sys/conf.h, and use it together with UID_ROOT
to define DRM_DEV_UID and DRM_DEV_GID in the drmP.h files.

So there is one place where the UID's and GID's are defined.

Submitted by: ed@
Reviewed by: ed@, dumbbell@
Differential Revision: https://reviews.freebsd.org/D3360