Set memory bandwidth limit to filter out resolutions above 720p@60Hz to
avoid underflow errors due to the bandwidth needs of higher resolutions.

am43xx can not provide enough bandwidth to DISPC to correctly handle
'high' resolutions.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>