psb_video: refine VP8 Decoding API

To follow proposed change/clarification for dec_va_vp8.h,
refine VP8 Decoding API. This fixes WebRTC poor quality
issue. 

Changes to dec_va.vp8.h are below,
1. Remove mb_skip_coeff from VAPictureParameterBUfferVP8
2. num_of_partitions count the control partition

BUG= BZ: 158760, JIRA: CHRMOS-1062
TEST="update psb_video and libmix, run test in CHRMOS-1062.
      The video should not show corruption and color
      artifacts"

Change-Id: I649907ce2cfd07685a4acf4d3f08f3575ffcf8a0
Signed-off-by: Tianmi Chen <tianmi.chen@intel.com>
Signed-off-by: Sameer Kibey <sameer.kibey@intel.com>