vga_switcheroo: Remove assumptions about registration/unregistration ordering

vga_switcheroo assumes that the handler will be registered before the
last client, otherwise switching will not be enabled. Likewise it's
assumed that the handler will not be unregistered without at least one
client also being unregistered, otherwise switching will remain enabled
despite no longer having a handler. These assumptions cannot be enforced
if the handler is in a separate driver from both clients, as with the
gmux found in Apple laptops. Remove this assumption.

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