drm/i915/gvt: Make elsp_dwords in the right order

The context descriptors in elsp_dwords are stored in a reversed order and
the definition of context descriptor is also reversed. The revesred stuff
is hard to be used and might cause misunderstanding. Make them in the right
oder for following code re-factoring.

Tested on my SKL NUC.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>