drm/sched: De-clutter drm_sched_init

Move work queue allocation into a helper for a more streamlined function
body.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250704130754.89935-1-tvrtko.ursulin@igalia.com