drm/i915: Introduce intel_hpd_pin function.

The idea is to have an unique place to decide the pin-port
per platform.

So let's create this function now without any functional
change. Just adding together code from hdmi and dp together.

v2: Add missing pin for port A.
v3: Fix typo on subject.
    Avoid behaviour change so add WARN_ON and return
    if port A on HDMI. (by DK).

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811182650.14327-2-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_hotplug.c

@@ -101,6 +101,32 @@ enum port intel_hpd_pin_to_port(enum hpd_pin pin)
 	}
 }
 
+/**
+ * intel_hpd_pin - return pin hard associated with certain port.
+ * @port: the hpd port to get associated pin
+ *
+ * Return pin that is associatade with @port and HDP_NONE if no pin is
+ * hard associated with that @port.
+ */
+enum hpd_pin intel_hpd_pin(enum port port)
+{
+	switch (port) {
+	case PORT_A:
+		return HPD_PORT_A;
+	case PORT_B:
+		return HPD_PORT_B;
+	case PORT_C:
+		return HPD_PORT_C;
+	case PORT_D:
+		return HPD_PORT_D;
+	case PORT_E:
+		return HPD_PORT_E;
+	default:
+		MISSING_CASE(port);
+		return HPD_NONE;
+	}
+}
+
 #define HPD_STORM_DETECT_PERIOD		1000
 #define HPD_STORM_REENABLE_DELAY	(2 * 60 * 1000)