xwayland: fix render node detection on FreeBSD
Major/minor numbers are a.. major (ha) source of pain in FreeBSD porting.
In this case, Xwayland was thinking that /dev/dri/card0 is already a render node,
because the st_rdev on FreeBSD was passing the Linux-style check,
and because of the assumption, acceleration would fail because
various ioctls like AMDGPU_INFO would be denied on the non-render node.