drm/sched: Move internal prototypes to internal header

Now that we have a header file for internal scheduler interfaces we can
move some more prototypes into it. By doing that we eliminate the chance
of drivers trying to use something which was not intended to be used.

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/20250221105038.79665-6-tvrtko.ursulin@igalia.com