GstPreset: fix enum properties
@slomo
Submitted by Sebastian Dröge Link to original bug (#764240)
Description
+++ This bug was initially created as a clone of Bug 763814 +++
GstPreset has the same problem. Resulting in things like "frame-packing=Automatic (use incoming video information)" (this is from x264enc) in the generated preset files. Changing that one is more tricky as we need to keep backwards compatibility...