drm/nouveau/gsp: split rm alloc handling out on its own

Split base RM_ALLOC handling out into its own module.

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

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>