drm/mm: Fix caching of leftmost node in the interval tree

When we descend the tree to find our slot, if we step to the right, we
are no longer the leftmost node.

Fixes: f808c13f ("lib/interval_tree: fast overlap detection")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com> for now.
Link: https://patchwork.freedesktop.org/patch/msgid/20180220093738.1461-1-chris@chris-wilson.co.uk