drm/i915/gvt: Refine the ring mmio list definition

To improve the readability, let's remove the hard code for each mmio
definition. The raw offset remained as a comment, which give us an
offset based view.

This refine is to make it convenient for new platform enabling.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>