OMAP: DSS2: DISPC: dispc_save_context() and dispc_restore_context() cleanup

Iterate over manager and overlay id's to shorten dispc_save_context() and
dispc_restore_context().

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