drm/ast: change resolution may cause screen blurred

commit 1a37bd823891568f8721989aed0615835632d81a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/ast/ast_mode.c

@@ -568,6 +568,7 @@ static int ast_crtc_do_set_base(struct drm_crtc *crtc,
 	}
 	ast_bo_unreserve(bo);
 
+	ast_set_offset_reg(crtc);
 	ast_set_start_address_crt1(crtc, (u32)gpu_addr);
 
 	return 0;