va: correct the description of segment id map buffer for vp9e

previous description limited the segment id block 8x8
it is not accurate and should support different block size which
could be set by picture parameter.

attention: this change will impact backward compatiblility,
considering no one is using it. we could ignore this side effect

Signed-off-by: Carl Zhang <carl.zhang@intel.com>

va/va_enc_vp9.h

@@ -595,8 +595,10 @@ typedef struct _VAEncMiscParameterTypeVP9PerSegmantParam {
  * \brief VP9 Block Segmentation ID Buffer
  *
  * The application provides a buffer of VAEncMacroblockMapBufferType containing
- * the initial segmentation id for each 8x8 block, one byte each, in raster scan order.
- * Rate control may reassign it.  For example, a 640x480 video, the buffer has 4800 entries.
+ * the initial segmentation id for each block, block size is specified by seg_id_block_size
+ * in VAEncPictureParameterBufferVP9  , one byte each, in raster scan order.
+ * Rate control may reassign it.  For example, a 640x480 video, seg_id_block_size is
+ * VA_SEGID_BLOCK_16X16 , the buffer has 1200 entries.
  * The value of each entry should be in the range [0..7], inclusive.
  * If segmentation is not enabled, the application does not need to provide it.
  */