Documentation: nouveau: Update GSP message queue kernel-doc reference

GSP message queue docs has been moved following RPC handling split in
commit 8a8b1ec5261f20 ("drm/nouveau/gsp: split rpc handling out on its
own"), before GSP-RM implementation is versioned in commit c472d828348caf
("drm/nouveau/gsp: move subdev/engine impls to subdev/gsp/rm/r535/").
However, the kernel-doc reference in nouveau docs is left behind, which
triggers htmldocs warnings:

ERROR: Cannot find file ./drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c
WARNING: No kernel-doc for file ./drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c

Update the reference.

Fixes: c472d828348c ("drm/nouveau/gsp: move subdev/engine impls to subdev/gsp/rm/r535/")
Fixes: 8a8b1ec5261f ("drm/nouveau/gsp: split rpc handling out on its own")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20250611020805.22418-2-bagasdotme@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>