drm/panthor: Add a PANTHOR_BO_SYNC ioctl

drm/panthor: Add a PANTHOR_BO_SYNC ioctl

This will be used by the UMD to synchronize CPU-cached mappings when
the UMD can't do it directly (no usermode cache maintenance instruction
on Arm32).

v2:
- Change the flags so they better match the drm_gem_shmem_sync()
  semantics

v3:
- Add Steve's R-b

v4:
- No changes

v5:
- Drop Steve's R-b (the semantics changes call for a new review)

v6:
- Drop ret initialization in panthor_ioctl_bo_sync()
- Bail out early in panthor_ioctl_bo_sync() if ops.count is zero
- Drop unused PANTHOR_BO_SYNC_OP_FLAGS definition

v7:
- Hand-roll the sync logic (was previously provided by gem_shmem)

v8:
- Collect R-b

Signed-off-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-5-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>