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

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.

Fixes: 8a1cc07578bf ("drm/panthor: Add GEM logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251107171214.1186299-1-boris.brezillon@collabora.com