h264dec: fix issue that poc of some clips should be negative.
h264dec: fix issue that poc of some clips should be negative.

1. Related clips: VWP2_TOSHIBA_E.264 CVWP3_TOSHIBA_E.264

2. The type definition of picture poc difinitaion should be
signed int but not unsigned int according to ITU spec D.2.7.
Specifically provides as follows:
Any value within in the range of -(31 power of 2) to (31 power of 2) - 1,
inclusive, may be assigned to the values of the variables TopFieldOrderCnt
and BottomFieldOrderCnt of the reference pictures that are not available
due to the random access operation. For example,the value 0 may be
assigned to these variables.

3. Bug occurs in VaapiDPBManager::bumpDPB(),if poc less then
0,then output picture with minimal poc might be error.
See code below:
/*
 * if (!foundPicture || foundPicture->m_POC > picture->m_POC)
 *     foundPicture = picture, frameIndex = i;
*/

Signed-off-by: Zhong Cong <fbc1588e6b9489a864c9ee5f55f730ef2ee5e810@intel.com>

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