module: use "modprobe -r" instead of rmmod: GH-565

With the introduction of nvidia-uvm module, unloading nvidia module will fail unless nvidia-uvm is unloaded.  GH-565 suggests using "remove" or "softdep" modprobe.d clauses to teach modprobe to handle that for us; "rmmod" does not have that functionality, so switch to "modprobe -r".