OMAPDSS: fix regression with display sysfs files

omapdss's sysfs directories for displays used to have 'name' file,
giving the name for the display. This file was later renamed to
'display_name' to avoid conflicts with i2c sysfs 'name' file. Looks like
at least xserver-xorg-video-omap3 requires the 'name' file to be
present.

To fix the regression, this patch creates new kobjects for each display,
allowing us to create sysfs directories for the displays. This way we
have the whole directory for omapdss, and there will be no sysfs file
clashes with the underlying display device's sysfs files.

We can thus add the 'name' sysfs file back.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: NeilBrown <neilb@suse.de>