OMAPDSS: output: increase refcount in find_output funcs

Now that omap_dss_output has been combined into omap_dss_device, we can
add ref counting for the relevant output functions also.

This patch adds omap_dss_get_device() calls to the various find_output()
style functions. This, of course, means that the users of those
find_output functions need to do a omap_dss_put_device() after use.

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