When changing the fd, reset our grab state to ungrabbed

When changing the fd, reset our grab state to ungrabbed

Previously, calling grabbing a device after changing the fd was a no-op
because libevdev's grab state didn't match the fd:

libevdev_grab(LIBEVDEV_GRAB);
  .. fd is grabbed
  .. internal state is 'grabbed'
libevdev_change_fd();
  .. new fd is ungrabbed
  .. internal state is 'grabbed'
libevdev_grab(LIBEVDEV_GRAB);
  .. argument matches internal state and we exit without grabbing the device

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