OMAPFB: find best mode from edid

Use the new read_edid() function to get EDID information from the
display (when available), and use the information to use a suitable mode
at initialization time.

Hot-plug is not yet supported, so the timings selected at init time will
stay even if the monitor would be changed.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>