libavcodec/vp8dec: fix the multi-thread HWAccel decode error

Fix the issue: https://github.com/intel/media-driver/issues/317

the root cause is update_dimensions will be called multple times
when decoder thread number is not only 1, but update_dimensions
call get_pixel_format in each decode thread will trigger the
hwaccel_uninit/hwaccel_init more than once. But only one hwaccel
should be shared with all decode threads.
in current context,
there are 3 situations in the update_dimensions():
1. First time calling. No matter single thread or multithread,
   get_pixel_format() should be called after dimensions were
   set;
2. Dimention changed at the runtime. Dimention need to be
   updated when macroblocks_base is already allocated,
   get_pixel_format() should be called to recreate new frames
   according to updated dimension;
3. Multithread first time calling. After decoder init, the
   other threads will call update_dimensions() at first time
   to allocate macroblocks_base and set dimensions.
   But get_pixel_format() is shouldn't be called due to low
   level frames and context are already created.

In this fix, we only call update_dimensions as need.

Signed-off-by: Wang, Shaofei <shaofei.wang@intel.com>
Reviewed-by: Jun, Zhao <jun.zhao@intel.com>
Reviewed-by: Haihao Xiang <haihao.xiang@intel.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/vp8.c

@@ -187,7 +187,7 @@ static av_always_inline
 int update_dimensions(VP8Context *s, int width, int height, int is_vp7)
 {
     AVCodecContext *avctx = s->avctx;
-    int i, ret;
+    int i, ret, dim_reset = 0;
 
     if (width  != s->avctx->width || ((width+15)/16 != s->mb_width || (height+15)/16 != s->mb_height) && s->macroblocks_base ||
         height != s->avctx->height) {
@@ -196,9 +196,12 @@ int update_dimensions(VP8Context *s, int width, int height, int is_vp7)
         ret = ff_set_dimensions(s->avctx, width, height);
         if (ret < 0)
             return ret;
+
+        dim_reset = (s->macroblocks_base != NULL);
     }
 
-    if (!s->actually_webp && !is_vp7) {
+    if ((s->pix_fmt == AV_PIX_FMT_NONE || dim_reset) &&
+         !s->actually_webp && !is_vp7) {
         s->pix_fmt = get_pixel_format(s);
         if (s->pix_fmt < 0)
             return AVERROR(EINVAL);