drm/i915/gvt: correct the ggtt valid bit check in pipe control command

GGTT valid bit in pipe control command move to DWORD1 after SNB, so
change the valid check code correspondingly.

v2:
per Zhenyu's comment, replace the bit check with MACRO define
PIPE_CONTROL_GLOBAL_GTT_IVB

Signed-off-by: Yulei Zhang <yulei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>