gpu: nova-core: gsp: move Cmdq's DMA handle to a struct member

gpu: nova-core: gsp: move Cmdq's DMA handle to a struct member

The command-queue structure has a `dma_handle` method that returns the
DMA handle to the memory segment shared with the GSP. This works, but is
not ideal for the following reasons:

- That method is effectively only ever called once, and is technically
  an accessor method since the handle doesn't change over time,
- It feels a bit out-of-place with the other methods of `Cmdq` which
  only deal with the sending or receiving of messages,
- The method has `pub(crate)` visibility, allowing other driver code to
  access this highly-sensitive handle.

Address all these issues by turning `dma_handle` into a struct member
with `pub(super)` visibility. This keeps the method space focused, and
also ensures the member is not visible outside of the modules that need
it.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260319-b4-cmdq-dma-handle-v1-1-57840b4a4f90@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>