OMAPDSS: panel-n8x0: register the DSS driver after SPI probe

Register the DSS driver after SPI probe. This simplifies the
initialization. This is similar to what is being done e.g.
in panel-acx565akm.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>