drm/nouveau: consider CLASS_DISPLAY_3D devices while detecting dsm/optimus

The present code assumes that optimus is present whenever two
VGA (PCI_CLASS_DISPLAY_VGA) devices are present. This does not
seem to be the case of newer laptops with optimus, in which
case the nvidia gpu is a PCI_CLASS_DISPLAY_3D device.

Rework the logic so that we count both VGA and 3D devices,
when contemplating if optimus is present on the platform.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70208
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>