drm/panthor: Flush shmem writes before mapping buffers CPU-uncached

In the Linux kernel, the following vulnerability has been resolved: drm/panthor: Flush shmem writes before mapping buffers CPU-uncached The shmem layer zeroes out the new pages using cached mappings, and if we don't CPU-flush we might leave dirty cachelines behind, leading to potential data leaks and/or asynchronous buffer corruption when dirty cachelines are evicted.