Leave it entirely up to the driver whether to map prefetchable.

We have only a few drivers that pass BUS_SPACE_MAP_PREFETCHABLE to
pci_mapreg_map -- dev/pci/if_hme_pci.c, dev/pci/igma.c,
dev/pci/radeonfb.c, dev/pci/wcfb.c -- and they all do it for ROM or
framebuffers.

Based on a subthread from macallan@ in the earlier discussion:

https://mail-index.NetBSD.org/tech-kern/2017/03/23/msg021685.html

The thrust is that:
- a driver not asking for prefetchable mappings shouldn't get it
- some devices are correctly used with prefetchable mappings, but for
  some reason fail to set the prefetchable bit in the BAR
- nobody could identify any classes of device for which
  (a) the driver asks for prefetchable mappings, but
  (b) certain matching devices can't actually be used with
      prefetchable mappings

This brings pci_mapreg_map in line with what the documentation says;
the documentation never advertised that the prefetchable bit in the
BAR could cause the bus_space mapping to be prefetchable.

(riastradh)