drm/i915/gvt: avoid unnecessary vgpu switch

It's no need to switch vgpu if next vgpu is the same with current
vgpu, otherwise it will make performance drop in some case.

v2: correct the comments.

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