drm/i915: Put the kernel_context in drm_i915_private next to its friends

Just move the kernel_context member of drm_i915_private next to the
engines it is associated with.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-7-git-send-email-chris@chris-wilson.co.uk