vga_switcheroo: Don't require handler init callback

This callback is a no-op in nouveau, and the upcoming apple-gmux
switcheroo support won't require it either. Rather than forcing drivers
to stub it out, just make it optional and remove the callback from
nouveau.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>