drm/nouveau/fb/gp102-ga100: switch to simpler vram size detection method

Also exposes this for use by upcoming GSP-RM initialisation code.

v2: add SPDX header

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/20230525003106.3853741-3-skeggsb@gmail.com