drm/i915/bdw: Add a context and an engine pointers to the ringbuffer

Any given ringbuffer is unequivocally tied to one context and one engine.
By setting the appropriate pointers to them, the ringbuffer struct holds
all the infromation you might need to submit a workload for processing,
Execlists style.

v2: Drop ring->ctx since that looks terribly ill-defined for legacy
ringbuffer submission.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com> (v1)
Acked-by: Damien Lespiau <damien.lespiau@intel.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -134,6 +134,7 @@ int intel_lr_context_deferred_create(struct intel_context *ctx,
 		return ret;
 	}
 
+	ringbuf->ring = ring;
 	ringbuf->size = 32 * PAGE_SIZE;
 	ringbuf->effective_size = ringbuf->size;
 	ringbuf->head = 0;

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1594,6 +1594,7 @@ static int intel_init_ring_buffer(struct drm_device *dev,
 	INIT_LIST_HEAD(&ring->active_list);
 	INIT_LIST_HEAD(&ring->request_list);
 	ringbuf->size = 32 * PAGE_SIZE;
+	ringbuf->ring = ring;
 	memset(ring->semaphore.sync_seqno, 0, sizeof(ring->semaphore.sync_seqno));
 
 	init_waitqueue_head(&ring->irq_queue);

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -90,6 +90,8 @@ struct intel_ringbuffer {
 	struct drm_i915_gem_object *obj;
 	void __iomem *virtual_start;
 
+	struct intel_engine_cs *ring;
+
 	u32 head;
 	u32 tail;
 	int space;