drm/nouveau/disp/gf119-: avoid creating non-existent heads

We assume that each board has 4 heads for GF119+. However this is not
necessarily true - in the case of a GP108 board, the register indicated
that there were only 2.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101601


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>