drm/i915: Disable DRRS on IVB/HSW port != A

Currently we allow DRRS on IVB PCH ports, but we're missing a
few programming steps meaning it is guaranteed to not work.
And on HSW DRRS is not supported on anything but port A ever
as only transcoder EDP has the M2/N2 registers (though I'm
not sure if HSW ever has eDP on any other port).

Starting from BDW all transcoders have the dynamically
reprogrammable M/N registers so DRRS could work on any
port.

Stop initializing DRRS on ports where it cannot possibly work.

Cc: [email protected]
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Jani Nikula <[email protected]>
(cherry picked from commit f0d4ce5)
Signed-off-by: Tvrtko Ursulin <[email protected]>

Author: vsyrjala

drivers/gpu/drm/i915/display/intel_drrs.c

@@ -405,6 +405,7 @@ intel_drrs_init(struct intel_connector *connector,
 		struct drm_display_mode *fixed_mode)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_encoder *encoder = connector->encoder;
 	struct drm_display_mode *downclock_mode = NULL;
 
 	INIT_DELAYED_WORK(&dev_priv->drrs.work, intel_drrs_downclock_work);
@@ -416,6 +417,13 @@ intel_drrs_init(struct intel_connector *connector,
 		return NULL;
 	}
 
+	if ((DISPLAY_VER(dev_priv) < 8 && !HAS_GMCH(dev_priv)) &&
+	    encoder->port != PORT_A) {
+		drm_dbg_kms(&dev_priv->drm,
+			    "DRRS only supported on eDP port A\n");
+		return NULL;
+	}
+
 	if (dev_priv->vbt.drrs_type != SEAMLESS_DRRS_SUPPORT) {
 		drm_dbg_kms(&dev_priv->drm, "VBT doesn't support DRRS\n");
 		return NULL;