video: Fix possible leak in of_get_videomode()

In case videomode_from_timings() fails in function of_get_videomode(), the
allocated display timing data is not freed in the exit path. Make sure that
display_timings_release() is called in any case. Detected by Coverity CID
1309681.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>