drm/nv50: rewrite display irq handler

The previous handler basically worked correctly for a full-blown mode
change.  However, it did nothing at all when a partial (encoder only)
reconfiguation was necessary, leading to the display hanging on certain
types of mode switch.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_drv.h

@@ -610,6 +610,11 @@ struct drm_nouveau_private {
 	struct backlight_device *backlight;
 
 	struct nouveau_channel *evo;
+	struct {
+		struct dcb_entry *dcb;
+		u16 script;
+		u32 pclk;
+	} evo_irq;
 
 	struct {
 		struct dentry *channel_root;