drm/nouveau/kms/nv50-: Get rid of bogus nouveau_conn_mode_valid()

Ville also pointed out that I got a lot of the logic here wrong as well, whoops.
While I don't think anyone's likely using 3D output with nouveau, the next patch
will make nouveau_conn_mode_valid() make a lot less sense. So, let's just get
rid of it and open-code it like before, while taking care to move the 3D frame
packing calculations on the dot clock into the right place.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: d6a9efec

 ("drm/nouveau/kms/nv50-: Share DP SST mode_valid() handling with MST")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>