drm/i915/gvt: access shadow ctx via its virtual address directly

as shadow context is pinned in intel_vgpu_setup_submission() and
unpinned in intel_vgpu_clean_submission(), its base virtual address of
is safely obtained from lrc_reg_state. no need to call kmap()/kunmap()
repeatedly.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200415035728.26424-1-yan.y.zhao@intel.com