drm/i915: Reuse legacy breadcrumbs + tail emission

As GEN6+ is now a simple variant on the basic breadcrumbs + tail write,
reuse the common code.

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/1469432687-22756-26-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-17-git-send-email-chris@chris-wilson.co.uk