OMAPDSS: DISPC: clean up dispc_mgr_timings_ok

dispc_mgr_timings_ok() is a bit confusing how it handles the return
value.

Change the function to just return immediately when a timing is deemed
not valid, making the code much easier to follow.

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