drm/ast: change resolution may cause screen blurred

The value of pitches is not correct while calling mode_set.
The issue we found so far on following system:
- Debian8 with XFCE Desktop
- Ubuntu with KDE Desktop
- SUSE15 with KDE Desktop

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Cc: <stable@vger.kernel.org>
Tested-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>