drm/sched/tests: Use one lock for fence context

There is no need for separate locks for single jobs and the entire
scheduler. The dma_fence context can be protected by the scheduler lock,
allowing for removing the jobs' locks. This simplifies things and
reduces the likelyhood of deadlocks etc.

Replace the jobs' locks with the mock scheduler lock.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250527101029.56491-2-phasta@kernel.org