drm/nouveau/gsp: change gv100_gsp from global to static

Smatch reports this issue
gv100.c:46:1: warning: symbol 'gv100_gsp' was not declared. Should it be static?

gv100_gsp is only used in gv100.c so change its
storage-class specifier to static.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421133028.724954-1-trix@redhat.com