OMAPDSS: DISPC: add dispc_ovl_check()

This patch adds a new function, dispc_ovl_check(), which can be used to
verify scaling configuration for an overlay. The function gets both the
overlay and overlay manager as parameters, so that the caller does not
need to configure the hardware before using this function.

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