drm/amdkfd: remove redundant kfd_event_waiter.input_index

This always identical with the index of the event_waiter in the array.
No need to store it in the waiter record.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>