OMAPDSS: don't use dss bus in suspend/resume

We have support functions to suspend and resume all the displays that
are used with system suspend. These functions use the dss bus to iterate
the display devices.

As we aim to remove the custom dss bus totally, this patch removes the
explicit use of dss bus from these functions. Instead the
for_each_dss_dev() macro is used to go through the devices.

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