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: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-7-chris@chris-wilson.co.uk