drm/nouveau/fb/ga102-: construct vidmem heap via new gp102 paths

No need for custom implementation anymore.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230525003106.3853741-4-skeggsb@gmail.com