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RING_HEAD is no longer being updated (due to inhibiting context save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

drivers/gpu/drm/i915/intel_lrc.c

@@ -447,13 +447,6 @@ static void inject_preempt_context(struct intel_engine_cs *engine)
 
 	GEM_BUG_ON(engine->execlists.preempt_complete_status !=
 		   upper_32_bits(ce->lrc_desc));
-	GEM_BUG_ON(!IS_ALIGNED(ce->ring->size, WA_TAIL_BYTES));
-
-	memset(ce->ring->vaddr + ce->ring->tail, 0, WA_TAIL_BYTES);
-	ce->ring->tail += WA_TAIL_BYTES;
-	ce->ring->tail &= (ce->ring->size - 1);
-	ce->lrc_reg_state[CTX_RING_TAIL+1] = ce->ring->tail;
-
 	GEM_BUG_ON((ce->lrc_reg_state[CTX_CONTEXT_CONTROL + 1] &
 		    _MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT |
 				       CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT)) !=