drm/panthor: Simplify group idleness tracking

drm/panthor: Simplify group idleness tracking

csg_slot_sync_queues_state_locked() queries the queues state which can
then be used to determine if a group is idle or not. Let's base our
idleness detection logic solely on the {idle,blocked}_queues masks to
avoid inconsistencies between the group state and the state of its
subqueues.

v2:
- Add R-b

v3:
- Collect R-b

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: https://patch.msgid.link/20251128094839.3856402-2-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>