OMAPDSS: APPLY: Constify timings argument in dss_mgr_set_timings

The function dss_mgr_set_timings is supposed to apply timings passed by an
interface driver. It is not supposed to change the timings. Add const qualifier
to the omap_video_timings pointer argument in dss_mgr_set_timings().

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