drm/nouveau: rename "argc" to what it represents in GSP RPC routines

The name "argc" 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 "argc" to what it represents. Add terms in the decoder section to
explain their meaning.

No functional change is intended.

Signed-off-by: Zhi Wang <zhiw@nvidia.com>
[ Fix indentation. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-7-zhiw@nvidia.com