drm/i915/buddy: document the unused header bits

drm/i915/buddy: document the unused header bits

The largest possible order is (63-PAGE_SHIFT), given that our min chunk
size is PAGE_SIZE. With that we should only need at most 6 bits to
represent all possible orders, giving us back 4 bits for other potential
uses.  Include a simple selftest to verify this.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210126103019.177622-1-matthew.auld@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>