drm/etnaviv: Protect the scheduler's pending list with its lock

drm/etnaviv: Protect the scheduler's pending list with its lock

Commit 704d3d60fec4 ("drm/etnaviv: don't block scheduler when GPU is still
active") ensured that active jobs are returned to the pending list when
extending the timeout. However, it didn't use the pending list's lock to
manipulate the list, which causes a race condition as the scheduler's
workqueues are running.

Hold the lock while manipulating the scheduler's pending list to prevent
a race.

Cc: stable@vger.kernel.org
Fixes: 704d3d60fec4 ("drm/etnaviv: don't block scheduler when GPU is still active")
Reported-by: Philipp Stanner <phasta@kernel.org>
Closes: https://lore.kernel.org/dri-devel/964e59ba1539083ef29b06d3c78f5e2e9b138ab8.camel@mailbox.org/
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250602132240.93314-1-mcanal@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>