drivers/nouveau/kms/nv50-: Reject format modifiers for cursor planes

Nvidia hardware doesn't actually support using tiling formats with the
cursor plane, only linear is allowed. In the future, we should write a
testcase for this.

Fixes: c586f30b

 ("drm/nouveau/kms: Add format mod prop to base/ovly/nvdisp")
Cc: James Jones <jajones@nvidia.com>
Cc: Martin Peres <martin.peres@free.fr>
Cc: Jeremy Cline <jcline@redhat.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: James Jones <jajones@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>