merge the changes from drm 85b9f737db0d2a845e4d7e2bbf9ad12ff9e2227c

merge the changes from drm 85b9f737db0d2a845e4d7e2bbf9ad12ff9e2227c
with our changes and the work recently done by Arto Huusko
<arto.huusko@pp2.inet.fi> and FUKAUMI Naoki <fun@naobsd.org>.

it includes all the changes arto provided from both mesa-drm and
the r6xx-r7xx-support branch.  it does not yet include code to
handle the (deleted) drm_pciids.h file, but i'll probably just
check in a generated one for now.

i have not yet merged the changes from outside this dir.

from arto's messages to tech-x11:

The important change that was needed is that drm_scatter.c was
fixed to return pointer to all allocated pages, not just the
beginning of the allocated segments.

Other changes:

  - drm_scatter maps COHERENT memory
  - drm_drawable: drawable handle allocation is done
    inside lock
  - drm_memory: when mapping "agp" memory, store offset
    of mapped area, so that new requests to same offset
    return the same area instead of trying to remap
    and fail
  - drm_vm: use bus_space_mmap for frame buffer and registers

  - r600_cp.c: ioremapfree allocated gart range
  - radeon_cp.c: use mtsleep
  - some memset calls I had added had their args swapped,
    and no memory was cleared