drm/omap: dss: Add support for reporting memory bandwidth limitation

The get_memory_bandwidth_limit() in dispc_ops can be used to query the
memory bandwidth limit of dispc by upper layers.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>