fbdev: intelfb: Fix potential divide by zero

Variable var->pixclock is controlled by user and can be assigned
to zero. Without proper check, divide by zero would occur in
intelfbhw_validate_mode and intelfbhw_mode_to_hw.

Error out if var->pixclock is zero.
Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/intelfb/intelfbdrv.c

@@ -1222,6 +1222,9 @@ static int intelfb_check_var(struct fb_var_screeninfo *var,
 
 	dinfo = GET_DINFO(info);
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	/* update the pitch */
 	if (intelfbhw_validate_mode(dinfo, var) != 0)
 		return -EINVAL;