probe: ProbePointsHelper adjustable minimum points

Factor out minimum required points check for ProbePointsHelper
in prep for setting different values for quad_gantry_level
and z_tilt_adjust

Signed-off-by: John "Fess" Fessenden <fess@fess.org>

klippy/extras/quad_gantry_level.py

@@ -14,6 +14,7 @@ class QuadGantryLevel:
         self.printer.register_event_handler("klippy:connect",
                                             self.handle_connect)
         self.probe_helper = probe.ProbePointsHelper(config, self.probe_finalize)
+        self.probe_helper.minimum_points(3)
         gantry_corners = config.get('gantry_corners').split('\n')
         try:
             gantry_corners = [line.split(',', 1)