libavcodec/dvbsub: fix for Display Definition Segment

Given ETSI EN 300 743 V1.3.1 (2006-11), 7.2.1 Display definition segment
display window parameters are given in this order XMin, XMax, YMin, YMax
if  display_window_flag is set, but here this is not the case.
As a consequence the DVB subtitles are not displayed upon some videos.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>