By definition (enum __drm_capabilities), cases other than CAP_SYS_ADMIN
aren't possible. Add in a KASSERT safety belt and return false in
!INVARIANTS cases if an invalid value is passed in, as it would be a
programmer error.
This fixes a -Wreturn-type error with gcc 5.3.0.
MFC after: 1 week
Reported by: devel/amd64-gcc (5.3.0)
Sponsored by: EMC / Isilon Storage Division