edid: Set timings flags according to edid

Timing flags are never set, so they may contain garbage. Since some
drivers check them, video output may be broken on those drivers.

Initialize them to 0 and check for few common flags.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Simon Glass <sjg@chromium.org>