drm/i915: Use a define for the default priority [0]

Explicitly assign the default priority, and give it a name. After much
discussion, we have chosen to call it I915_PRIORITY_NORMAL!
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-7-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_context.c

@@ -199,6 +199,7 @@ __create_hw_context(struct drm_i915_private *dev_priv,
 	kref_init(&ctx->ref);
 	list_add_tail(&ctx->link, &dev_priv->context_list);
 	ctx->i915 = dev_priv;
+	ctx->priority = I915_PRIORITY_NORMAL;
 
 	/* Default context will never have a file_priv */
 	ret = DEFAULT_CONTEXT_HANDLE;

drivers/gpu/drm/i915/i915_gem_request.h

@@ -70,6 +70,7 @@ struct i915_priotree {
 	struct rb_node node;
 	int priority;
 #define I915_PRIORITY_MAX 1024
+#define I915_PRIORITY_NORMAL 0
 #define I915_PRIORITY_MIN (-I915_PRIORITY_MAX)
 };