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