OMAPDSS: MANAGER: Create a function to check manager timings

Create a function dss_mgr_check_timings() which wraps around the function
dispc_mgr_timings_ok(). This is mainly a clean up to hide dispc functions
from interface drivers.

dss_mgr_check_timings() is added in the function dss_mgr_check(), it currently
takes the timings maintained in the omap_dss_device struct. This would be later
replaced by the timings stored in the manager's private data.

Make dss_mgr_check_timings() and dispc_mgr_timings_ok() take a const
omap_video_timings pointer since these functions just check the timings.

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