drm/nouveau/gsp: split rpc handling out on its own

Later patches in the series add HALs around various RM APIs in order to
support a newer version of GSP-RM firmware.  In order to do this, begin
by splitting the code up into "modules" that roughly represent RM's API
boundaries so they can be more easily managed.

Aside from moving the RPC function pointers, no code change is indended.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>