Gfx-Display: fixed a potential panic issue.

Gfx-Display: fixed a potential panic issue.

BZ: 17268

When HDMI flip chain isn't initialized, de-queue buffer from it would
cause panic as its buffer number may be 0.

Fixed by checking whether the flip chain is initialized, before getting
the buffer handle.

Change-Id: I61d9f17bd484c352b2039a61b99b6947f7c36347
Signed-off-by: Austin Hu <austin.hu@intel.com>
Signed-off-by: Tong Bo <box.tong@intel.com>
Reviewed-on: http://android.intel.com:8080/27729
Reviewed-by: Xu, Randy <randy.xu@intel.com>
Tested-by: Xu, Randy <randy.xu@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>