drm-shim: Let the driver choose to overwrite the first render node.

drm-shim: Let the driver choose to overwrite the first render node.

When I was writing drm-shim, I was focused on the v3d kmsro case -- use my
intel device as the kmsro display device and add on a simulator-based v3d
device that we could render with.  But for the noop backends we use for
shader-db, it's a lot more useful to just overwrite the first render node
in the system so that you don't have to pass a -d <how many render nodes I
already have in my system> argument.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4664>