Graphics: Refine the PVRSRVGetDCFrontBufferKM to avoid duplicates blt

Graphics: Refine the PVRSRVGetDCFrontBufferKM to avoid duplicates blt

BZ: 33328

In previous solution, PVRSRVGetDCFrontBufferKM would return the last
handled buffer in flip command queues. There is a delay between handling
the flip command and eglSwapBuffers(), there are in different threads,
so in some case, the old buffer would be used in PVR2DBltWidi();
Current solution is to return the last buffer used in eglSwapBuffers(),
Since in current pvr dirvers, eglSwapBuffers() would wait for the
buffer rendering complete, then the buffer can be returned for Widi.

Change-Id: Iaf65746c23c276ba0f21ceb929661a5a6d042c59
Signed-off-by: gwang23 <gang.a.wang@intel.com>
Reviewed-on: http://android.intel.com:8080/46657
Reviewed-by: Tao, Tao Q <tao.q.tao@intel.com>
Reviewed-by: Tong, BoX <box.tong@intel.com>
Tested-by: Tong, BoX <box.tong@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>