drm/i915/gvt: add missing display part reset for vGPU reset

We also need reset vGPU virtual display emulation. Since all vreg has
been cleared, we need reset display related vreg to reflect our display
setting.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>