drm/panthor: Fix panthor_gpu_coherency_set()

drm/panthor: Fix panthor_gpu_coherency_set()

GPU_COHERENCY_PROTOCOL takes one of GPU_COHERENCY_xx
not BIT(GPU_COHERENCY_xx).

v3:
- New commit

v4:
- Add Steve's R-b

v5:
- No changes

v6:
- No changes

v7:
- No changes

v8:
- No changes

Cc: Akash Goel <akash.goel@arm.com>
Fixes: dd7db8d911a1 ("drm/panthor: Explicitly set the coherency mode")
Reported-by: Steven Price <steven.price@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-3-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>