h264dec: fix long term ref picture mark as unused for reference bug.

SPEC: 8.2.5.4.4
All pictures for which LongTermFrameIdx is greater than
max_long_term_frame_idx_plus1-1 and that are marked as
"used for long-term reference" are  marked as "unused for reference".

VaapiPictureH264 use int32_t for longTermFrameIdx,
but uint32_t is used for longTermFrameIdx in execRefPicMarkingAdaptive1().
when some clips have nagtive longTermFrameIdx(-1) and mmco equals to 4,
No long term ref picture will be marked as "unused for reference",
for example: MR3_TANDBERG_B.264.

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