drm/i915/gvt: Add one missing MMIO handler for D_SKL_PLUS

_PLANE_CTL_3_A, _PLANE_CTL_3_B and _PLANE_SURF_3_A are handled, but
miss _PLANE_SURF_3_B.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200601030457.14002-1-colin.xu@intel.com