drm/amdkfd: Clean up kfd_wait_on_events

Cleaned up the code while resolving some potential bugs and
inconsistencies in the process.

Clean-ups:
* Remove enum kfd_event_wait_result, which duplicates
  KFD_IOC_EVENT_RESULT definitions
* alloc_event_waiters can be called without holding p->event_mutex
* Return an error code from copy_signaled_event_data instead of bool
* Clean up error handling code paths to minimize duplication in
  kfd_wait_on_events

Fixes:
* Consistently return an error code from kfd_wait_on_events and set
  wait_result to KFD_IOC_WAIT_RESULT_FAIL in all failure cases.
* Always call free_waiters while holding p->event_mutex
* copy_signaled_event_data might sleep. Don't call it while the task state
  is TASK_INTERRUPTIBLE.

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