We already have a fallback in place to fill out the unique from
dev->unique, which is set to something reasonable in drm_dev_alloc.

Which means we only need to have a special set_busid for pci devices,
to be able to care the backwards compat code for drm 1.1 around, which
libdrm still needs.

While developing and testing this patch things blew up in really
interesting ways, and the code is rather confusing in naming things
between the kernel code, ioctl #defines and libdrm. For the next brave
dragon slayer, document all this madness properly in the userspace
interface section of gpu.tmpl.

v2: Make drm_dev_set_unique static and update kerneldoc.

v3: Entire rewrite, plus document what's going on for posterity in the
gpu docbook uapi section.

v4: Drop accidental amdgpu hunk (Emil).

v5: Drop accidental omapdrm vblank counter change (Emil).

v6: Rebase on top of the sphinx conversion.

Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk> (virt_gpu)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>