common: edid: check for digital display earlier

When searching for detailed timing in EDID, check for digital display
earlier. There is no point parsing other parameters if this flag is not
present.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>