va*enc: use upstream pts when possible

The va*encs compute their pts and dts based on their own reference set by gst_va_base_enc_start(), which propagates to the base video encoder via gst_video_encoder_set_min_pts(), which configures a time_adjustment when necessary.

In some cases (e.g. RFC 7273), we want the pts to keep the capture time, which was lost due to the above.

This commit uses the 1000 h reference only if the first buffer pts is not already large enough to guarantee dts will always be positive. For NTP or PTP clocks, the initial capture time can now be kept.