drm/tilcdc: Clear bits of register we're going to set.

Bits weren't cleared so resolution changes didn't work.
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Darren Etheridge <detheridge@ti.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/tilcdc/tilcdc_crtc.c

@@ -299,6 +299,8 @@ static int tilcdc_crtc_mode_set(struct drm_crtc *crtc,
 	 * a value of 0 as 1
 	 */
 	if (priv->rev == 2) {
+		/* clear bits we're going to set */
+		reg &= ~0x78000033;
 		reg |= ((hfp-1) & 0x300) >> 8;
 		reg |= ((hbp-1) & 0x300) >> 4;
 		reg |= ((hsw-1) & 0x3c0) << 21;