vo_gpu_next: don't spam render queue underrun logs on frame->still

If we are redrawing on a still image (e.g. ass events), then the pl
queue will hit the PL_QUEUE_MORE case which is fine but there's no need
to spam the log here. The information may be useful for when an actual
video is being played, but when a frame is still (e.g. paused or only a
single image) it's just noise.

Fixes #17840.