drm/i915/gvt: remove useless function

drm/i915/gvt: remove useless function

Fix the following clang warning:

drivers/gpu/drm/i915/gvt/gtt.c:590:20: warning: unused function
'ppgtt_set_guest_root_entry' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1618294728-78952-1-git-send-email-jiapeng.chong@linux.alibaba.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>