drm/amdgpu: move amdgpu_ctx_mgr_entity_fini to f_ops flush hook (V4)

With this we can now terminate jobs enqueue into SW queue the moment
the task is being killed instead of waiting for last user of
drm file to release it.

Also stop checking for kref_read(&ctx->refcount) == 1 when
calling drm_sched_entity_do_release since other task
might still hold a reference to this entity but we don't
care since KILL means terminate job submission regardless
of what other tasks are doing.

v2:
Use returned remaining timeout as parameter for the next call.
Rebase.

v3:
Switch to working with jiffies.
Streamline remainder TO usage.
Rebase.

v4:
Rebase.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>