va: Do not use a common parent_class in vabasedec.

We have only one copy of gst_va_base_dec_parent_class inside the vabasedec, so it can not handle the case when there are multi va decoders inside one pipeline. The pipeline:

gst-launch-1.0 filesrc location=xxx.h264 ! h264parse
! vah264dec ! msdkh265enc ! vah265dec ! fakesink

generates a assertion of

"invalid cast from 'GstVaH264Dec' to 'GstH265Decoder"

and gets a crash.

We should keep the parent_class for each decoder type.

sys/va/gstvaav1dec.c

@@ -80,9 +80,6 @@ struct _GstVaAV1Dec
   gboolean need_negotiation;
 };
 
-#define parent_class gst_va_base_dec_parent_class
-extern gpointer gst_va_base_dec_parent_class;
-
 /* *INDENT-OFF* */
 static const gchar *src_caps_str = GST_VIDEO_CAPS_MAKE_WITH_FEATURES ("memory:VAMemory",
             "{ NV12, P010_10LE }") " ;" GST_VIDEO_CAPS_MAKE ("{ NV12, P010_10LE }");
@@ -136,7 +133,8 @@ gst_va_av1_dec_negotiate (GstVideoDecoder * decoder)
   GST_INFO_OBJECT (self, "Negotiated caps %" GST_PTR_FORMAT,
       base->output_state->caps);
 
-  return GST_VIDEO_DECODER_CLASS (parent_class)->negotiate (decoder);
+  return GST_VIDEO_DECODER_CLASS (GST_VA_BASE_DEC_GET_PARENT_CLASS
+      (decoder))->negotiate (decoder);
 }
 
 static GstCaps *
@@ -871,7 +869,7 @@ static void
 gst_va_av1_dec_dispose (GObject * object)
 {
   gst_va_base_dec_close (GST_VIDEO_DECODER (object));
-  G_OBJECT_CLASS (parent_class)->dispose (object);
+  G_OBJECT_CLASS (GST_VA_BASE_DEC_GET_PARENT_CLASS (object))->dispose (object);
 }
 
 static void