fbdev: da8xx-fb: add missing regulator_disable() in fb_probe

fbdev: da8xx-fb: add missing regulator_disable() in fb_probe

The error handling code in fb_probe misses regulator_disable if
regulator_enable is called successfully. The previous commit only
adds regulator_disable in the .remove(), forgetting the error
handling code in the .probe.

Fix this by adding a new error label to call regulator_disable.

Fixes: 611097d5daea("fbdev: da8xx: add support for a regulator")
Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Helge Deller <deller@gmx.de>