drm/nouveau: factor out r535_gsp_msgq_recv_one_elem()

Prepare for supporting receive the large GSP RPC message.

Factor out r535_gsp_msgq_recv_one_elem(). Fold its params into a data
structure of params. Move the allocation of the GSP RPC message to its
caller. Refine the variable names in the re-factor.

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/20250124182958.2040494-14-zhiw@nvidia.com