drm/nouveau: rename the variable "cmd" to "msg" in r535_gsp_cmdq_{get, push}()

Refine the name to align with the terms in the kernel doc.

No functional change is intended.

Signed-off-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-12-zhiw@nvidia.com