video: xilinxfb: Simplify error path

devm_iounmap is called automatically that's why remove it from the code
dev_set_drvdata(dev, NULL) is called by generic code
after device_release or on probe failure.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>