OMAPDSS: DISPC: handle 0 out_width/out_height in ovl_setup()

Overlay out_width/height are set to 0 when scaling is not used by the
users of omapdss. Currently ovl_setup() expects the caller of
ovl_setup() to convert those zero values to width or height.

This patch makes ovl_setup() accept zero values for out_width/height,
making calling ovl_setup() a bit simpler as the overlay_info can be
just passed to this function without modifications.

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