drm: Add NULL check about irq functions

The struct drm_driver has some function pointers for irq. They are
gpu specific and some functions aren't essential things. This can
prevents creation of unnecessary dummy function for irq.
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -291,10 +291,13 @@ static void drm_irq_vgaarb_nokms(void *cookie, bool state)
 	if (!dev->irq_enabled)
 		return;
 
-	if (state)
+	if (state) {
+		if (dev->driver->irq_uninstall)
 			dev->driver->irq_uninstall(dev);
-	else {
+	} else {
+		if (dev->driver->irq_preinstall)
 			dev->driver->irq_preinstall(dev);
+		if (dev->driver->irq_postinstall)
 			dev->driver->irq_postinstall(dev);
 	}
 }
@@ -338,6 +341,7 @@ int drm_irq_install(struct drm_device *dev)
 	DRM_DEBUG("irq=%d\n", drm_dev_to_irq(dev));
 
 	/* Before installing handler */
+	if (dev->driver->irq_preinstall)
 		dev->driver->irq_preinstall(dev);
 
 	/* Install handler */
@@ -363,7 +367,9 @@ int drm_irq_install(struct drm_device *dev)
 		vga_client_register(dev->pdev, (void *)dev, drm_irq_vgaarb_nokms, NULL);
 
 	/* After installing handler */
+	if (dev->driver->irq_postinstall)
 		ret = dev->driver->irq_postinstall(dev);
+
 	if (ret < 0) {
 		mutex_lock(&dev->struct_mutex);
 		dev->irq_enabled = 0;
@@ -416,6 +422,7 @@ int drm_irq_uninstall(struct drm_device *dev)
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		vga_client_register(dev->pdev, NULL, NULL, NULL);
 
+	if (dev->driver->irq_uninstall)
 		dev->driver->irq_uninstall(dev);
 
 	free_irq(drm_dev_to_irq(dev), dev);