drm/i915/gvt: add support to command SWTESS_BASE_ADDRESS

This cmd access is found on BDW.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200305055413.6833-1-yan.y.zhao@intel.com