power_domain: Handle power_domain set_state error

While transitioning a power domain to a particular state, we should not
expect any driver input if the driver has failed to initiate a power
domain transition.

Change-Id: If9b03221b30251f01855e220e5ef2a80f5c7ef80
Signed-off-by: Raphael Gault <raphael.gault@arm.com>