OMAPDSS: Apply manager timings instead of direct DISPC writes

Replace the function dispc_mgr_set_timings() with dss_mgr_set_timings() in the
interface drivers. The latter function ensures that the timing related DISPC
registers are configured according to the shadow register programming model.

Remove the call to dispc_mgr_go() in dpi_set_timings() as the manager's go bit
is set by dss_mgr_set_timings().

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