drm/bridge: lt9611: rework the mode_set function

The mode_set callback is deprectated for drm_bridges in favour of using
atomic_enable callback. Move corresponding code into the function
lt9611_bridge_atomic_enable() and turn lt9611_bridge_pre_enable() into
the proper atomic_pre_enable callback.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>