drm/nouveau: fix the broken marco GSP_MSG_MAX_SIZE

The macro GSP_MSG_MAX_SIZE refers to another macro that doesn't exist.
It represents the max GSP message element size.

Fix the broken marco so it can be used to replace some magic numbers in
the code.

Signed-off-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-8-zhiw@nvidia.com