media: mgb4: Enumerate only the available timings

Enumerate only the available (as given by the sysfs setup - our "EDID
replacement") timings, not all theoretically possible. This is the video
outputs part of the previous inputs patch that somehow got "lost in
translation".

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>