OMAPDSS: DISPC: Create helper function dispc_mgr_is_lcd()

Create a helper function called dispc_mgr_is_lcd() which returns true if the
manager is LCD or LCD2.

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