st/va: force to flush the last p frame in idr period

st/va: force to flush the last p frame in idr period

During dual instance encoding submission, if the second encode task and first
encode task have no reference dependency, e.g. p following with idr-frame,
there is a chance the second task will use for its reconstructed picture
buffer the same buffer used by first task for its reference/reconstructed
picture. In this case, buffer corruption may occur depending on encoding
speed. Fix is to force flush these two tasks separately to avoid race condition

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=98005
Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>