vah264dec: shaking image

Video played via vah264dec on my AMD APU is "shaking" very badly in latest gstreamer master (it worked fine on 1.18). It happens on every video. I did a little investigation and this is the first commit that caused this: 734e2a74

Also if I go to gstvapool.c and remove vpool->starting = TRUE; inside gst_va_pool_start() method it works correctly which means that probably the logic under G_UNLIKELY inside gst_va_pool_release_buffer() is causing this. Moving GST_BUFFER_FLAGS (buffer) = 0; after parent class call seems to fix this, but I dunno if this is the right thing to do.

This is how played video is "shaking": vah264dec-problem