OMAPDSS: DISPC: Constify omap_overlay_info in dispc_ovl_setup()

The struct omap_overlay_info passed to dispc_ovl_setup() is used to configure
DISPC registers. It shouldn't modify the overlay_info structure. The pos_y field
was being changed in dispc_ovl_setup in the case of interlaced displays. Fix
this and const qualifier to the omap_overlay_info argument.

Signed-off-by: Archit Taneja <archit@ti.com>