edid: Add HDMI flag to timing info

Some DVI monitors don't show anything in HDMI mode since audio stream
confuses them. To solve this situation, this commit adds HDMI flag in
timing data and sets it accordingly during edid parsing.

First existence of extension block is checked. If it exists and it is
CEA861 extension, then data blocks are checked for presence of HDMI
vendor specific data block. If it is present, HDMI flag is set.

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