mfvideoenc: Re-define default GOP size value

Merged Seungha Yang requested to merge seungha.yang/gst-plugins-bad:mfvideoenc-gop-size into master
The behavior for zero AVEncMPVGOPSize value would be
varying depending on GPU vendor implementation and some
GPU will produce keyframe only once at the beginning of encoding.
That's unlikely expected result for users.

To make this property behave consistently among various GPUs,
this commit will change default value of "gop-size" property to -1
which means "auto". When "gop-size" is unspecified, then
mfvideoenc will calculate GOP size based on framerate
like that of our x264enc implementation.

See also
Edited by Seungha Yang

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