drm/sched: Move drm_sched_entity_is_ready to internal header

Helper is for scheduler internal use so lets hide it from DRM drivers
completely.

At the same time we change the method of checking whethere there is
anything in the queue from peeking to looking at the node count.

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>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250221105038.79665-5-tvrtko.ursulin@igalia.com