OMAPDSS: remove omap_dss_start/stop_device()

The omap_dss_start_device() and omap_dss_stop_device(), called by the
DSS output drivers, are old relics. They originally did something
totally else, but nowadays they increase the module ref count for panels
that are enabled.

This model is quite broken: the panel modules may be used even before
they are enabled. For example, configuring the panel requires calls to
functions located in the panel modules.

In the following patches we try to improve the ref count management for
the modules and display devices. The first step, however, is to remove
the omap_dss_start/stop_device() totally.

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