drm/nouveau/disp: drop unused argument in nv50_dp_mode_valid

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230628212248.3798605-2-kherbst@redhat.com