Move update() and sync() from omap_dss_device to omap_dss_driver.

Also, update was hardcoded to use virtual channel 0. This patch adds a
parameter that specifies the VC.

This is part of a larger patch-set, which moves the control from omapdss
driver to the display driver.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>