drm/panthor: Fix the full_tick check

drm/panthor: Fix the full_tick check

We have a full tick when the remaining time to the next tick is zero,
not the other way around. Declare a full_tick variable so we don't get
that test wrong in other places.

v2:
- Add R-b

v3:
- Collect R-b

Fixes: de8548813824 ("drm/panthor: Add the scheduler logical block")
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: https://patch.msgid.link/20251128094839.3856402-4-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>