drm/i915: Allocate context objects from stolen

As we never expose context objects directly to userspace, we can forgo
allocating a first-class GEM object for them and prefer to use the
limited resource of reserved/stolen memory for them. Note this means
that their initial contents are undefined.

However, a downside of using stolen objects for execlists is that we
cannot access the physical address directly (thanks MCH!) which prevents
their use.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -157,6 +157,8 @@ i915_gem_alloc_context_obj(struct drm_device *dev, size_t size)
 	struct drm_i915_gem_object *obj;
 	int ret;
 
+	obj = i915_gem_object_create_stolen(dev, size);
+	if (obj == NULL)
 		obj = i915_gem_alloc_object(dev, size);
 	if (obj == NULL)
 		return ERR_PTR(-ENOMEM);

drivers/gpu/drm/i915/intel_lrc.c

@@ -1845,7 +1845,7 @@ int intel_lr_context_deferred_create(struct intel_context *ctx,
 
 	context_size = round_up(get_lr_context_size(ring), 4096);
 
-	ctx_obj = i915_gem_alloc_context_obj(dev, context_size);
+	ctx_obj = i915_gem_alloc_object(dev, context_size);
 	if (IS_ERR(ctx_obj)) {
 		ret = PTR_ERR(ctx_obj);
 		DRM_DEBUG_DRIVER("Alloc LRC backing obj failed: %d\n", ret);