drm/nouveau: refine the variable names in r535_gsp_rpc_push()

The variable names in r535_gsp_rpc_push() are quite confusing and some
of them are not representing what they really are.

Update the names and explanations in the decoder section of the
kernel doc. Refine the names 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/20250124182958.2040494-10-zhiw@nvidia.com