xserver: default to libinput if no driver is found
Make xorg-server default to try the xf86-input-libinput driver if a
configured driver can't be found. This only applies if a specific driver
for an input device has been configured manually in xorg, but that driver
can't be found when starting xorg.
Discussed with: manu, jbeich
Approved by: ports-secteam (joenum)
85fd72e11d