octeontx2-pf: Dereference only a valid pointer

commit 782463ee26fceceae04adaf4b84be77f2684815e from
git@git.assembla.com:cavium/WindRiver.linux.git

iommu_domain can be NULL hence check for non
NULL pointer before dereferencing it.

Change-Id: Iff01276a67f35954f1556b731d43339d152e268d
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/20065


Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <Sunil.Goutham@cavium.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>