Enable PCI BAR reallocation by default.

When pci_realloc_bars was first added, the intention was to eventually
enable it by default, but it was left disabled to preserve existing
behavior. The setting is pretty conservative in that it does not
attempt to allocate resources for BARs that the BIOS/firmware leaves
disabled. It only attempts to reallocate resources for a BAR that the
firmware programmed during boot but that conflicts with another
resource during the kernel's device scan.

PR 221350 is an example of a machine that this knob fixes.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D18965