drm/i915: Simplify i915_gtt_color_adjust()

If we remember that node_list is a circular list containing the fake
head_node, we can use a simple list_next_entry() and skip the NULL check
for the allocated check against the head_node.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161216074718.32500-3-chris@chris-wilson.co.uk