drm/i915: WARN if power sequencer is not connected to the LVDS port on pre-ilk

We don't support using the power sequencer with other ports besides LVDS
on pre-ilk platforms. WARN if someone has mistakenly connected the power
sequencer to the wrong port.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-7-ville.syrjala@linux.intel.comReviewed-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -1236,7 +1236,12 @@ void assert_panel_unlocked(struct drm_i915_private *dev_priv, enum pipe pipe)
 		pp_reg = PP_CONTROL(pipe);
 		panel_pipe = pipe;
 	} else {
+		u32 port_sel;
+
 		pp_reg = PP_CONTROL(0);
+		port_sel = I915_READ(PP_ON_DELAYS(0)) & PANEL_PORT_SELECT_MASK;
+
+		WARN_ON(port_sel != PANEL_PORT_SELECT_LVDS);
 		intel_lvds_port_enabled(dev_priv, LVDS, &panel_pipe);
 	}