gpu: nova-core: gsp: replace BootUnloadGuard with local handlers

When adding the GSP unload capability, we introduced `BootUnloadGuard`
to automatically call `Gsp::unload` whenever an error occurred during
the boot process, in order to try to reset the GSP to a valid state.

This approach is not well-suited to the errors that may occur in HALs:
by definition, an error occurring in the HAL means that the GSP is not
booted; yet the first thing that `Gsp::unload` does is queue a shutdown
message to the GSP, which will inevitably result in a timeout when done
from a HAL.

Furthermore, `BootUnloadGuard` is problematic because it holds
additional references to the boot context, notably the `Falcon`s. These
extra references stand in the way of making some of the `Falcon`'s
methods mutable, since those methods would require exclusive access. As
this behavior is only needed in one place, introducing dedicated types
for it is distracting and unnecessary.

Thus, remove `BootUnloadGuard` and adopt a two-level error handling
strategy:

- HALs are free to handle their errors as they see fit (most likely, by
  running their unload bundle if it is ready by the time of the error),
- `Gsp::boot` uses a `ScopeGuard` that runs `Gsp::unload`, since the
  GSP should be up and running by the time `GspHal::boot` has returned.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260709-nova-bootcontext-v6-3-520cbf8b9b50@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

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