iommu/omap: Remove unnecessary null pointer check

The pointer obj is dereferenced in line 146 and 149 respectively, so it is not
necessary to check null again in line 149 and 175. And I have checked that all
the callers of these two functions guarantee the parameter obj passed is not
null.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Acked-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>