drm/nouveau/kms/nv50-: guard against enabling cursor on disabled heads

commit 697bb728 upstream.

Userspace has started doing this, which upsets the display class hw
error checking in various unpleasant ways.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>