Fix radeon framebuffer mmapping.

The framebuffer is not a system-RAM-backed, TTM-managed object;
rather, it is a VRAM-backed object.  So we have to use its bus
address, which is a little less sketchy with bus_space_mmap anyway.
Defer real radeon attachment until mountroot so we can load firmware.
Rename radeondrmkms to just plain radeon.

radeondrmkmsfb will stay as is until the name radeonfb is available
(currently taken by legacy dev/pci/radeonfb.c).
Report the same PCI domain (cf_unit) as everything else in NetBSD.
link against libdrm_radeon, from @riastradh
enable KMS on x86.
Recover from channel halt errors by using the 3 strikes xacterr rule.

Adapted from the RaspberryPI linux driver.

This allows the rum(4) I was sent to be somewhat usable. Need to
investigate further what is causing the problem in the first place.

Might help PR/49019 (RPI: interrupt storm when url0 is up)
Don't use saved EREG registers because sljit 0.91 can generate
bogus code on amd64. The A and X registers are saved on the stack.

The most recent version of sljit fixes bogus code generation but
it's not backward compatible with sljit 0.91.
Fix non debug build.
Sync VM_MAXUSER_ADDRESS and VM_MAX_ADDRESS with other hp300 derived ports.

There is no particular reason to have different values among m68k ports.
Briefly checked on LUNA-II.
Reorganize and bump VM related constants.

- sync VM_MAXUSER_ADDRESS and VM_MAX_ADDRESS with other hp300 derived ports
  and use same definitions for USRSTACK as other m68k ports
  (no worth to have sun3/sunos compat in these days)
- remove unused KUSER_AREA
- bump MAXTSIZ, MAXDSIZ, and MAXSSIZ

These changes allow my 64MB TT030 build pkgsrc/textproc/icu
which seems to require >200MB VA space.
(note our current 040/060 pmap implementation can't handle >224MB VA size)
Use tvtohz instead of ad-hacking it myself, suggested by apb@.
Fix __raw_i915_read64 on i386.  From rjs@.
Avoid needless #ifdef __i386__ with judicious use of MIN.
npf_conn_conkey: fix a comment.
Improved the NORMAL_GLOBALS macro in the sun4u case, so only the relevant bit (AG) in %pstate is affected - ok martin@
Move sun3 bootloader address to allow loading growing GENERIC kernels.
Shrink GENERIC kernel to fit to bootloader's limit (2MB).

Maybe now we can't call it GENERIC, but it still boots on TME.
Comment out options and devices to sync GENERIC rev 1.90.
Move CONFIG_X86 from files.i915drmkms to drm_os_netbsd.h.