OMAPDSS: implement display sysfs without dss bus

We aim to remove the custom omapdss bus totally, as it's quite a strange
construct and won't be compatible with common display framework. One
problem on the road is that we have sysfs files for each display, and
they depend on the omapdss bus.

This patch creates the display sysfs files independent of the omapdss
bus. This gives us backwards compatibility without using the omapdss bus
for the sysfs files.

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