drm/nv04/crtc: Bail out if FB is not bound to crtc

This commit resolves a possible 'NULL pointer dereference'
It uses the same approach as radeon, intel and nouveau/nv50

Fixes bug 'Nouveau: Kernel oops when unplugging external monitor'
https://bugs.freedesktop.org/show_bug.cgi?id=40336Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv04_crtc.c

@@ -781,11 +781,20 @@ nv04_crtc_do_mode_set_base(struct drm_crtc *crtc,
 	struct drm_device *dev = crtc->dev;
 	struct drm_nouveau_private *dev_priv = dev->dev_private;
 	struct nv04_crtc_reg *regp = &dev_priv->mode_reg.crtc_reg[nv_crtc->index];
-	struct drm_framebuffer *drm_fb = nv_crtc->base.fb;
-	struct nouveau_framebuffer *fb = nouveau_framebuffer(drm_fb);
+	struct drm_framebuffer *drm_fb;
+	struct nouveau_framebuffer *fb;
 	int arb_burst, arb_lwm;
 	int ret;
 
+	NV_DEBUG_KMS(dev, "index %d\n", nv_crtc->index);
+
+	/* no fb bound */
+	if (!atomic && !crtc->fb) {
+		NV_DEBUG_KMS(dev, "No FB bound\n");
+		return 0;
+	}
+
+
 	/* If atomic, we want to switch to the fb we were passed, so
 	 * now we update pointers to do that.  (We don't pin; just
 	 * assume we're already pinned and update the base address.)
@@ -794,6 +803,8 @@ nv04_crtc_do_mode_set_base(struct drm_crtc *crtc,
 		drm_fb = passed_fb;
 		fb = nouveau_framebuffer(passed_fb);
 	} else {
+		drm_fb = crtc->fb;
+		fb = nouveau_framebuffer(crtc->fb);
 		/* If not atomic, we can go ahead and pin, and unpin the
 		 * old fb we were passed.
 		 */