drm/sched: Fix description of drm_sched_stop

Since commit 222b5f04 ("drm/sched: Refactor ring mirror list
handling."), drm_sched_hw_job_reset is no longer there, so let's adjust
the doc comment accordingly.

Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>