ARM: OMAP2+: Powerdomain: Fix unchecked dereference of arch_pwrdm

Commit 'cd8abed' "ARM: OMAP2+: Powerdomain: Remove the need to
always have a voltdm associated to a pwrdm" leads to the following
Smatch complaint:

arch/arm/mach-omap2/powerdomain.c:131 _pwrdm_register()
	 error: we previously assumed 'arch_pwrdm' could be null (see line 105)

So, fix the unchecked dereference of arch_pwrdm.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>