drm/nouveau: use correct conflicting framebuffer API

nouveau was calling the fbdev API which has issues with modules
and built-ins. Call the correct API.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Fixes: 2dd4d163 ("drm/nouveau: remove open-coded version of remove_conflicting_pci_framebuffers()")
Link: https://lore.kernel.org/lkml/21b52c28-3ace-cd13-d8ce-f38f2c6b2a96@infradead.org/T/#u


Signed-off-by: Dave Airlie <airlied@redhat.com>