gpu: nova-core: send UNLOADING_GUEST_DRIVER GSP command upon unloading

gpu: nova-core: send UNLOADING_GUEST_DRIVER GSP command upon unloading

Currently, the GSP is left running after the driver is unbound. This is
not great for several reasons, notably that it can still access shared
memory areas that the kernel will now reclaim (especially problematic on
setups without an IOMMU).

Fix this by sending the `UNLOADING_GUEST_DRIVER` GSP command when the
`Gpu` is dropped. This stops the GSP and lets us proceed with the rest
of the unbind sequence in a later patch.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Co-developed-by: Eliot Courtney <ecourtney@nvidia.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260529-nova-unload-v7-2-678f39209e00@nvidia.com
[acourbot: `Result<()>` -> `Result`]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>