drm/i915/gvt: use context lrc_reg_state for shadow ppgtt override

We can replace kmap by using context's lrc_reg_state directly
for shadow ppgtt table override.

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200506095948.124979-1-zhenyuw@linux.intel.com