omxvideoenc: Set force_keyframe to FALSE after handling it
omxvideoenc: Set force_keyframe to FALSE after handling it

There's no reason why the base class should forward the event
further downstream if we already handled it and will insert a
keyframe.

pliu6/gst-omx,varunkumara/test-repo,ismelykh/gst-omx-deinterlace,surround-io/gst-omx,freedesktop-unofficial-mirror/gstreamer__gst-omx,ismelykh/gst-omx-deinterlace,01org/gst-omx,varunkumara/test-repo,surround-io/gst-omx,zuh/gst-omx-rpi,zuh/gst-omx-rpi,changzhix/gst-omx,renesas-rcar/gst-omx,renesas-rcar/gst-omx,RidgeRun/gst-omx,pliu6/gst-omx,tjormola/gst-omx,renesas-devel/gst-omx,renesas-devel/gst-omx,CapOM/gst-omx,xuguangxin/gst-omx,varunkumara/test-repo,CapOM/gst-omx,CapOM/gst-omx,01org/gst-omx,renesas-rcar/gst-omx,tjormola/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx,changzhix/gst-omx,xuguangxin/gst-omx,freedesktop-unofficial-mirror/gstreamer__gst-omx,leio/gst-omx,leio/gst-omx,GStreamer/gst-omx,RidgeRun/gst-omx