drm/panfrost: Add a PANFROST_SYNC_BO ioctl

drm/panfrost: Add a PANFROST_SYNC_BO 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:
- Add more to the commit message
- Change the flags to better match the drm_gem_shmem_sync semantics

v3:
- Add Steve's R-b

v4:
- No changes

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

v6:
- Bail out early in panfrost_ioctl_sync_bo() if op_count is zero

v7:
- Hand-roll our own bo_sync() helper

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/20251208100841.730527-11-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>