sunxi: Fix display timing flags

flags member of struct timing was not initialized,
this took to unpredictable behaviour of display flags,
such DISPLAY_FLAGS_HSYNC_HIGH instead of _LOW etc.

Init timing->flags = 0

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>