OMAPDSS: fix set_timings

set_timings function of DSS's output drivers are not consistent. Some of
them disable the output, set the timings, and re-enable the output. Some
set the timings on the fly, while the output is enabled. And some just
store the given timings, so that they will be taken into use next time
the output is enabled.

We require the DISPC output to be disabled when changing the timings,
and so we can change all the output drivers' set_timings to just store
the given timings.

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