drm: Convert drm_vma_manager to embedded interval-tree in drm_mm

Having added an interval-tree to struct drm_mm, we can replace the
auxiliary rb-tree inside the drm_vma_manager with it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470236651-678-2-git-send-email-chris@chris-wilson.co.uk