gpu: nova-core: replace `Duration` with `Delta`

The kernel's `Delta` type was not available when the `wait_on` function
was introduced. Now that it is, switch to it as it is more compact than
`Duration` and cannot panic.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250624-nova-delta-v1-1-b37d75a593ac@nvidia.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>