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

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

If we want to be able to skip CPU cache maintenance operations on
CPU-cached mappings, the UMD needs to know the kind of coherency
in place. Add a field to drm_panthor_gpu_info to do that. We can re-use
a padding field for that since this object is write-only from the
KMD perspective, and the UMD should just ignore it.

v2:
- New commit

v3:
- Make coherency protocol a real enum, not a bitmask
- Add BUILD_BUG_ON()s to make sure the values in panthor_regs.h and
  those exposed through the uAPI match

v4:
- Add Steve's R-b

v5:
- No changes

v6:
- No changes

v7:
- Fix kernel doc

v8:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Karunika Choo <karunika.choo@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-4-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>