If the backend doesn't have a DRM FD, fallback to the renderer's.
This accomodates for the situation where the headless backend hasn't
picked a DRM FD in particular, but the renderer has picked one.
If we aren't trying to create a dumb buffer allocator, and if the
DRM device has a render node (ie, not a split render/display SoC),
then we can use the render node instead of the primary node. This
should allow wlroots to run under seatd when the current user
doesn't have the permission to open primary nodes (logind has a
quirk to allow physically logged in users to open primary nodes).
