drm/sched: Fix comment in drm_sched_run_job_work()

drm/sched: Fix comment in drm_sched_run_job_work()

drm_sched_run_job_work() contains a comment which explains that an
entity being NULL means that there is no more work to do. It can,
however, also mean that there is work, but the scheduler doesn't have
enough credits to process the jobs right now.

Provide this detail in the comment.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20251028134602.94125-3-phasta@kernel.org