drm/i915/execlists: Remove the ring advancement under preemption

Load an empty ringbuffer for preemption, ignoring the lite-restore
workaround as we know the preempt context is always idle before preemption.

Note that after some digging by Michal Winiarski, we found that
RING_HEAD is no longer being updated (due to inhibiting context save
restore) so this patch is already in effect!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221133236.29402-1-chris@chris-wilson.co.uk