xwayland: fix render node detection on FreeBSD

Val Packett requested to merge valpackett/xserver:xw-fbsd into master

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.

/cc @jbeich @manu

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