drm/amdkfd: Handle remaining BUG_ONs more gracefully v2

In most cases, BUG_ONs can be replaced with WARN_ON with an error
return. In some void functions just turn them into a WARN_ON and
possibly an early exit.

v2:
* Cleaned up error handling in pm_send_unmap_queue
* Removed redundant WARN_ON in kfd_process_destroy_delayed

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