drm/i915: Rename request->list to link for consistency

We use "list" to denote the list and "link" to denote an element on that
list. Rename request->list to match this idiom.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-14-git-send-email-chris@chris-wilson.co.uk