vaapi: Implement device-only setup

In this case, the user only supplies a device and the frame context
is allocated internally by lavc.

(cherry picked from commit 5dd9a4b88b)

libavcodec/vaapi_decode.h

@@ -69,6 +69,9 @@ typedef struct VAAPIDecodeContext {
 
     AVHWFramesContext    *frames;
     AVVAAPIFramesContext *hwfc;
+
+    enum AVPixelFormat    surface_format;
+    int                   surface_count;
 } VAAPIDecodeContext;