drm/nouveau: rename "argv" to what it represents on the GSP message send path

The name "argv" has different meanings in different functions.

To improve the readability, it's better to refine it to a name that
reflects what it represents.

Rename "repc" to what it represents in the GSP message send path.
Wrap the long container_of() into to_gsp_hdr() to make checkpatch.pl
happy.

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/20250124182958.2040494-4-zhiw@nvidia.com