evdev: improve type-safety on dispatch switches

evdev: improve type-safety on dispatch switches

Set the dispatch type on creation, then check that whenever we try to get the
dispatch struct. This avoids a potential mismatch between the backends.

Plus, use of container_of means we're not dependent on the exact layout
anymore.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>