drm: rcar-du: Use the VBK interrupt for vblank events

BugLink: http://bugs.launchpad.net/bugs/1756121

commit cbbb90b0 upstream.

When implementing support for interlaced modes, the driver switched from
reporting vblank events on the vertical blanking (VBK) interrupt to the
frame end interrupt (FRM). This incorrectly divided the reported refresh
rate by two. Fix it by moving back to the VBK interrupt.

Fixes: 906eff7f ("drm: rcar-du: Implement support for interlaced modes")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: thongsyho <thong.ho.px@rvc.renesas.com>
Signed-off-by: Nhan Nguyen <nhan.nguyen.yb@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/gpu/drm/rcar-du/rcar_du_crtc.c

@@ -531,7 +531,7 @@ static irqreturn_t rcar_du_crtc_irq(int irq, void *arg)
 	status = rcar_du_crtc_read(rcrtc, DSSR);
 	rcar_du_crtc_write(rcrtc, DSRCR, status & DSRCR_MASK);
 
-	if (status & DSSR_FRM) {
+	if (status & DSSR_VBK) {
 		drm_handle_vblank(rcrtc->crtc.dev, rcrtc->index);
 		rcar_du_crtc_finish_page_flip(rcrtc);
 		ret = IRQ_HANDLED;