drm/i915/gvt: Don't use ggtt_validdate_range() with size=0

Use vgpu_gmadr_is_valid() directly instead.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>