decklinkvideosrc: respect pixel format property even if mode is set to auto

Before this patch, if mode=auto and video-format!=auto, video-format would always be ignored, and get set to 8bit-yuv, or if detected to be RGB444, then it would be set to 8bit-argb. This change respects video-format if it is set to 10bit-yuv (v210) or 8bit-bgra, even when mode=auto.

Closes #772 (closed)