OMAPDSS: DSI: fix depopulating dsi peripherals

After v3.16-rc1, devices populated with of_platform_populate() should be
depopulated using of_platform_depopulate(). Failure to do so prevents
the devices from being populated again (e.g. when loading a module).

This patch fixes the problem for the OMAP DSI driver.

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