scmi_power_domain: Change order of sanity checks

Validation of the input parameters should ideally be done before
assessing the capability of the devices. Hence the current order of the
checks in the function scmi_pd_power_state_notify_handler is incorrect.
This commit fixes this order.

Change-Id: If0d00ae1c7bd98f29d0a5edefe13e54e10818fcb
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>