drm/vc4: crtc: Use local chan variable

The vc4_crtc_handle_page_flip already has a local variable holding the
value of vc4_crtc->channel, so let's use it instead.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/439c589baec72ddb89159857a2d078fdd77b02a2.1599120059.git-series.maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -533,7 +533,7 @@ static void vc4_crtc_handle_page_flip(struct vc4_crtc *vc4_crtc)
 		 * the CRTC and encoder already reconfigured, leading to
 		 * underruns. This can be seen when reconfiguring the CRTC.
 		 */
-		vc4_hvs_unmask_underrun(dev, vc4_crtc->channel);
+		vc4_hvs_unmask_underrun(dev, chan);
 	}
 	spin_unlock_irqrestore(&dev->event_lock, flags);
 }