Provide GstPreset for vaapi encoders.

Submitted by Hyunjun Ko @zzoon

Link to original bug (#783501)

Description

There's GstPreset supported in gstreamer.
If we support this for vaapi encoders, I expect this can be involved to gst-validate scenario for vaapi.(bug 773304)

Eg. If we have "Main" preset for main profile with dct8x8 disabled,

gst-launch-1.0 videotestsrc num-buffers=50 ! encodebin profile="video/quicktime,varient=iso:video/x-h264+Main" ! filesink location=test.mp4

and we can also include preset to gst-validate tools as the following:

gst-validate-transcoding-1.0 -o "video/quicktime,variant=iso;:video/x-h264+Main" [input] [output]