drm/nouveau/fb: add missing sysmen flush callbacks

Closes: https://gitlab.freedesktop.org/drm/nouveau/-/issues/203


Fixes: 5728d064 ("drm/nouveau/fb: handle sysmem flush page from common code")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230405110455.1368428-1-kherbst@redhat.com