drm/i915/gvt: do not let pin count of shadow mm go negative

shadow mm's pin count got increased in workload preparation phase, which
is after workload scanning.
it will get decreased in complete_current_workload() anyway after
workload completion.
Sometimes, if a workload meets a scanning error, its shadow mm pin count
will not get increased but will get decreased in the end.
This patch lets shadow mm's pin count not go below 0.

Fixes: 2707e44 ("drm/i915/gvt: vGPU graphics memory virtualization")
Cc: zhenyuw@linux.intel.com
Cc: stable@vger.kernel.org #4.14+
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/gtt.c

@@ -1946,7 +1946,7 @@ void _intel_vgpu_mm_release(struct kref *mm_ref)
  */
 void intel_vgpu_unpin_mm(struct intel_vgpu_mm *mm)
 {
-	atomic_dec(&mm->pincount);
+	atomic_dec_if_positive(&mm->pincount);
 }
 
 /**