nouveau: fix disp disabling with GSP

This func ptr here is normally static allocation, but gsp r535
uses a dynamic pointer, so we need to handle that better.

This fixes a crash with GSP when you use config=disp=0 to avoid
disp problems.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231222043308.3090089-4-airlied@gmail.com