OMAPDSS: clean up dss_mgr_set_timings

dss_mgr_set_timings() can only be called when the output is not active.
This means that most of the code in the function is extra, as there's no
need to write the values to registers, etc, because that will be handled
when the output will be enabled.

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