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

Submitted by Joshua M. Doe

Link to original bug (#796979)

Description

I got hung up for several hours trying to debug why I couldn't get 10-bit YUV (v210) from an SDI source via a Decklink Extreme 12G card. Turns out 8-bit YUV will always be used if the mode is set to AUTO. Eventually I noticed a warning about this issued in gstdecklinkvideosrc.cpp:1221, "Warning: mode=auto and format!=auto may not work" (I know, I should have noticed this warning in the debug output sooner).

I created a patch which seems to fix this, and might hopefully save future users some frustration