[gstreamer] Simplify viewfinder preview caps a bit upon construction. Contributes to JB#8597
[gstreamer] Simplify viewfinder preview caps a bit upon construction. Contributes to JB#8597

That's exactly what we did with for video caps (commit a7f0d17624490dc375a28bbf8d17993180633a67)

sailfishos/gst-droidcamsrc,sailfishos/gst-droidcamsrc,sailfishos/gst-droidcamsrc