vpu: encoder: small cleanup + h264 encoder now copies headers for keyframes

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
vpu: encoder: generate two I-frames for each force keyframe bit set

This fixes problems with h264parse; other encoders such as x264enc too
generate two I-frames instead of just one

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>