OMAPDSS: MANAGER: Update display sysfs store

The display sysfs attribute's store function needs to be changed with the
introduction of outputs.

The DSS driver ensures that there is one display per output, and that a
registered omap_dss_device will have an output connected to it. The display
sysfs store function unsets the current output connected to the manager, and
sets it with the output connected to the new display. If the new display doesn't
have an output for some reason, we just bail out. The function doesn't set/unset
output->device links. These remain the same as when the omap_dss_device was
registered.

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