GFX-Display: Avoid block in drm_wait_vblank().

GFX-Display: Avoid block in drm_wait_vblank().

BZ: 25652

When PipeB is power gated, we won't enable pipeb vblank interrupt
and wait vblank.

Change-Id: I02fbb83debc74e75fa7cac444acac3cb3d643809
Signed-off-by: Jason Hu <jason.hu@intel.com>
Signed-off-by: Jackie Li <yaodong.li@intel.com>
Reviewed-on: http://android.intel.com:8080/37190
Reviewed-by: Zhu, Tianyang <tianyang.zhu@intel.com>
Reviewed-by: Ding, Haitao <haitao.ding@intel.com>
Tested-by: Ding, Haitao <haitao.ding@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>