fbdev: da8xx-fb: Fix error handling in .remove()

Even in the presence of problems (here: regulator_disable() might fail),
it's important to unregister all resources acquired during .probe() and
disable the device (i.e. DMA activity) because even if .remove() returns
an error code, the device is removed and the .remove() callback is never
called again later to catch up.

This is a preparation for making platform remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>
Fixes: 611097d5 ("fbdev: da8xx: add support for a regulator")