drm/nouveau: nvkm_gsp_radix3_sg() should use nvkm_gsp_mem_ctor()

Function nvkm_gsp_radix3_sg() uses nvkm_gsp_mem objects to allocate the
radix3 tables, but it unnecessarily creates those objects manually
instead of using the standard nvkm_gsp_mem_ctor() function like the
rest of the code does.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202230608.1981026-2-ttabi@nvidia.com