drm/sched: Disallow initializing entities with no schedulers

drm/sched: Disallow initializing entities with no schedulers

Since we have removed the case where amdgpu was initializing entitites
with either no schedulers on the list, or with a single NULL scheduler,
and there appears no other drivers which rely on this, we can simplify the
scheduler by explicitly rejecting that early.

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: Christian König <christian.koenig@amd.com>
Acked-by: Philipp Stanner <phasta@kernel.org>
Tested-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260417103744.76020-2-tvrtko.ursulin@igalia.com