Handle attach failure and don't try a framebuffer in i915 on failure.
Expose the agp_i810 softc.

This is an Intel-specific kludge so that i915drmkms can get at the
GTT, graphics translation table, for pre-Sandy Bridge Intel graphics
devices.
First draft of i915_gem_gtt.c for pre-Sandy Bridge (gen<6) devices.

These go through the legacy AGP cruft to get at the GTT.
Oops -- define struct intel_gtt::do_idle_maps.

Missed this in last commit.
Kludgey attempt to fill in intel_gtt stubs.
Implement and use non-interruptible DRM_WAIT_* gizmos.
Expose agp_i810_write_gtt_entry for drm2.

Also add an agp_i810_post_gtt_entry to do a posting read for that GTT
entry.
Fix up i915_gem_gtt for agp-style gen<6 devices.

- Set the DMA mask correctly: gen<4 needs 32-bit; gen4+, 36-bit.
- Use agp_i810_write_gtt_entry, not AGP_BIND_PAGE.
- Do a posting read after changing all the GTT entries.
Take the vmobjlock in i915_gem_release_mmap around pmap_page_protect.