drm/nouveau: rename "repc" to "gsp_rpc_len" on the GSP message recv path

The name "repc" has different meanings in different contexts.

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

Rename "repc" to "gsp_rpc_len" in the GSP message recv path. Add an
section in the doc to explain the terms.

No functional change is intended.

Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-3-zhiw@nvidia.com