etnaviv: Add lock around pending_ctx

etnaviv: Add lock around pending_ctx

The content of rsc->pending_ctx could be changed from multiple contexts
and thus from multiple threads. The per-context lock is not sufficient
to protect this list. Add per-resource lock to protect this list.

Fixes: e5cc66dfad0 ("etnaviv: Rework locking")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6454>