dmar: Don't try to reserve PCI regions for non-existing devices

In some cases we might have to create DMAR context before the
corresponding device has been enumerated by the PCI bus.
In that case we get called with NULL dev, because of that trying
to reserve PCI regions causes a NULL pointer dereference in
pci_find_pcie_root_port.

Sponsored by: Stormshield
Obtained from: Semihalf
MFC after: 2 weeks
Reviewed by: kib, rlibby
Differential revision: https://reviews.freebsd.org/D32589