OMAP: DSS2: DSI: Remove extra iounmap in error path

iounmap is already done in label err2: so extra iounmap in
the error handling path could be removed.

Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>