drm/msm: Make irq_postinstall optional

Allow the KMS operation 'irq_postinstall' to be optional
so that the target display drivers don't need to define
a dummy function if they don't need one.

v3: No changes
Reviewed-by: NSean Paul <sean@poorly.run>
Signed-off-by: NJordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Signed-off-by: NRob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/msm_drv.c

@@ -752,7 +752,11 @@ static int msm_irq_postinstall(struct drm_device *dev)
 	struct msm_drm_private *priv = dev->dev_private;
 	struct msm_kms *kms = priv->kms;
 	BUG_ON(!kms);
-	return kms->funcs->irq_postinstall(kms);
+
+	if (kms->funcs->irq_postinstall)
+		return kms->funcs->irq_postinstall(kms);
+
+	return 0;
 }
 
 static void msm_irq_uninstall(struct drm_device *dev)