drm/panthor: Provide a custom dma_buf implementation

drm/panthor: Provide a custom dma_buf implementation

Before we introduce cached CPU mappings, we want a dma_buf
implementation satisfying synchronization requests around CPU
accesses coming from a dma_buf exported by our driver. Let's
provide our own implementation relying on the default
gem_shmem_prime helpers designed for that purpose.

v5:
- New patch

v6:
- Collect R-b

v7:
- Hand-roll the dma_buf sync/import logic (was previously done by
  generic prime/shmem helpers)

v8:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-2-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>