LinuxKPI: Support lazy BAR allocation

Linux KPIs like pci_resource_start/len assume that BARs have been
allocated, but FreeBSD lazily allocates BARs if it cannot allocate the
firmware-allocated BARs. Thus using the Linux KPIs must force allocation
of the BARs rather than returning 0 for the start and length, which can
crash drm-kmod drivers that assume the BARs are valid. This is needed
for the AMDGPU driver to be able to attach on SiFive's HiFive Unmatched.

Reviewed by: hselasky, jhb, mav
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32447