dmar: Disable PMR in driver attach routine

Previously it was disabled right before translation was enabled.
This way the disable logic is still executed even when translation
is not be activated, e.g. with hw.iommu.dma=0 tunable set.
On some platforms we need to disable PMR in order for core dump to work.
At the same time it was observed that enabling translation has
a significant impact on network performance.
With this patch PMR can be disabled, with IOMMU translation not being
turned on by appending the following to the loader.conf:

hw.dmar.enable=1
hw.dmar.pmr.disable=1
hw.dmar.dma=0

Sponsored by: Stormshield
Obtained from: Semihalf
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D34907