commit 202b52b7fbf7 ("drm: Track drm_mm nodes with an interval tree")
introduced a requirement that the special drm_mm.head_node was
initialised and marked as not being allocated. It is a very special node
that has no side but has a hole that represents the drm_mm address
space, and holds the list of nodes. Since it is not a real node, it is
not part of the node rbtree and we detect this as it being unallocated.
This presumed that drm_mm_init() was initialising it to zero. It happens
that i915 kzallocs its objects and so it was accidentally setting it,
but for generic use we cannot make that assumption.