firmware: arm_scmi: remove some unnecessary checks

The "pi->dom_info" buffer is allocated in init() and it can't be NULL
here.  These tests are sort of weird as well because if "pi->dom_info"
was NULL but "domain" was non-zero then it would lead to an Oops.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>