OMAP: DSS2: Taal: Use device name in backlight_device_register

Panel Taal driver uses the string "taal" to register for a backlight device.
This causes backlight_device_register() to fail when a second taal panel
is added. Use dev_name(&dssdev->dev) as a parameter instead of the string.

Note: This will break backlight related sysfs commands. Use the name as
generated by the DSS2 driver, in the form "displayi", which represents
the ith registered display device.

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