OMAP: DSS2: Fix omap_dss_probe() error path

Perform graceful cleanup on errors instead of just bailing out.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>