drm/panthor: Recover from panthor_gpu_flush_caches() failures

In the Linux kernel, the following vulnerability has been resolved:

drm/panthor: Recover from panthor_gpu_flush_caches() failures

We have seen a few cases where the whole memory subsystem is blocked
and flush operations never complete. When that happens, we want to:

- schedule a reset, so we can recover from this situation
- in the reset path, we need to reset the pending_reqs so we can send
new commands after the reset
- if more panthor_gpu_flush_caches() operations are queued after
the timeout, we skip them and return -EIO directly to avoid needless
waits (the memory block won't miraculously work again)

Note that we drop the WARN_ON()s because these hangs can be triggered
with buggy GPU jobs created by the UMD, and there's no way we can
prevent it. We do keep the error messages though.

v2:
- New patch

v3:
- Collect R-b
- Explicitly mention the fact we dropped the WARN_ON()s in the commit
message

v4:
- No changes