drm/fsl-dcu: remove regmap return value checks

It is not common to do regmap return value checks, especially not
for memory mapped device. We can rule out most error returns since
the conditions are static and we know they are ok (e.g. offset
aligned to register stride). Also without proper error handling
they are not really valuable for the user. Hence remove most of
them.

The check in the interrupt handler is worth keeping since a
volatile register won't be readable in case register caching is
still enabled.

Signed-off-by: Stefan Agner <stefan@agner.ch>