drm/i915: Fix ordering of sanitize ppgtt and sanitize execlists

The i915.enable_ppgtt option depends upon the state of
i915.enable_execlists option - so we need to sanitize execlists first.

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/1461932305-14637-2-git-send-email-chris@chris-wilson.co.uk