drm/i915/gtt: Don't try to clear failed empty pd allocation

When __gen8_ppgtt_alloc fails without allocating anything
we should not try to call __gen8_ppgtt_clear as there is
nothing to clear and underlying code will complain with:

[  157.861645] gen8_pd_range:881 GEM_BUG_ON(start >= end)
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719153322.10464-1-michal.wajdeczko@intel.com

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1116,7 +1116,7 @@ static int gen8_ppgtt_alloc(struct i915_address_space *vm,
 
 	err = __gen8_ppgtt_alloc(vm, i915_vm_to_ppgtt(vm)->pd,
 				 &start, start + length, vm->top);
-	if (unlikely(err))
+	if (unlikely(err && from != start))
 		__gen8_ppgtt_clear(vm, i915_vm_to_ppgtt(vm)->pd,
 				   from, start, vm->top);