drm/panthor: Recover from panthor_gpu_flush_caches() failures

Summary

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