drm/nouveau: remove the magic number in r535_gsp_rpc_push()

There has been a GSP_MSG_MAX_SIZE which represents the max size of a GSP
message element header. Use it instead of a magic number.

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/20250124182958.2040494-9-zhiw@nvidia.com