iris: Drop stale syncobj references in fence_server_sync

When calling glWaitSync (fence_server_sync), we added dependencies
in all batches (render and compute) on existing work.  Even if
applications don't use compute at all, they theoretically could,
so we record that the compute batch depends on the render batch.

But if the application truly doesn't use compute, or rarely uses
it, we ended up recording dependencies on _all_ previous render
batches, racking up a massive list of syncobjs.  Not only is this
pointless, it also meant that we never allowed the kernel to free
the underlying i915_request objects.

There are a number of solutions to this problem, but for now, we
take a simple one: when recording a new syncobj dependency, we
walk the list and see if any of them have already passed.  If so,
that dependency has been fulfilled.  We no longer need to track it,
and can simply drop it from the list, unreferencing the syncobj.

Android's SurfaceFlinger in particular was hitting this issue,
as it uses glWaitSync, doesn't typically use compute shaders,
and runs for long durations.

Thanks to Yang A Shi <yang.a.shi@intel.com> and
Kefei Yao <kefei.yao@intel.com> for their excellent work in
tracking down this issue!

Fixes: f459c56be6b ("iris: Add fence support using drm_syncobj")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Yang A Shi <yang.a.shi@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6463>