drm/i915/gvt: correct the reset logic

The current_vgpu will set to NULL after stopping the scheduler when
the reset is triggered by current vgpu, so here need change the
judgement condition for current vgpu detection.

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