mixvbp: refine VP8 VA parameter usage

BZ: 158760

To follow proposed change/clarification for dec_va_vp8.h,
refine VP8 VA parameter usage.

Changes to dec_va.vp8.h are below,
1. Remove mb_skip_coeff from VAPictureParameterBUfferVP8
2. num_of_partitions count the control partition

Change-Id: I38876a3ab6a37d09f96183a13a7630ffe9708a2f
Signed-off-by: Tianmi Chen <tianmi.chen@intel.com>
3 files changed