drm/panfrost: Expose the selected coherency protocol to the UMD

drm/panfrost: Expose the selected coherency protocol to the UMD

Will be needed if we want to skip CPU cache maintenance operations when
the GPU can snoop CPU caches.

v2:
- New commit

v3:
- Fix the coherency values (enum instead of bitmask)

v4:
- Fix init/test on coherency_features

v5:
- No changes

v6:
- Collect R-b

v7:
- No changes

v8:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-10-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>