drm: Simplify drmm_alloc_ordered_workqueue return

Rather than returning ERR_PTR or NULL on failure, replace the NULL
return with ERR_PTR(-ENOMEM). This simplifies error handling at the
caller. While here, add kernel documentation for
drmm_alloc_ordered_workqueue.

Cc: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://lore.kernel.org/r/20250702232831.3271328-2-matthew.brost@intel.com