drm/i915: Remove wrong warning from i915_gem_context_clean

commit e9f24d5fb7cf3628b195b18ff3ac4e37937ceeae
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Mon Oct 5 13:26:36 2015 +0100

    drm/i915: Clean up associated VMAs on context destruction

Introduced a wrong assumption that all contexts have a ppgtt
instance. This is not true when full PPGTT is not active so
remove the WARN_ON_ONCE from the context cleanup code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -138,7 +138,7 @@ static void i915_gem_context_clean(struct intel_context *ctx)
 	struct i915_hw_ppgtt *ppgtt = ctx->ppgtt;
 	struct i915_vma *vma, *next;
 
-	if (WARN_ON_ONCE(!ppgtt))
+	if (!ppgtt)
 		return;
 
 	WARN_ON(!list_empty(&ppgtt->base.active_list));