omxvideodec: simplify _find_nearest_frame
omxvideodec: simplify _find_nearest_frame

No need to make it more complicated and error prone than
necessary. Also give the function a gst_omx_video_dec prefix
to distinct it from the encoder function.

https://bugzilla.gnome.org/show_bug.cgi?id=724236

varunkumara/test-repo,surround-io/gst-omx,CapOM/gst-omx,GStreamer/gst-omx,varunkumara/test-repo,CapOM/gst-omx,CapOM/gst-omx,leio/gst-omx,GStreamer/gst-omx,ismelykh/gst-omx-deinterlace,varunkumara/test-repo,renesas-rcar/gst-omx,freedesktop-unofficial-mirror/gstreamer__gst-omx,GStreamer/gst-omx,freedesktop-unofficial-mirror/gstreamer__gst-omx,ismelykh/gst-omx-deinterlace,leio/gst-omx,pliu6/gst-omx,pliu6/gst-omx,renesas-rcar/gst-omx,renesas-rcar/gst-omx,surround-io/gst-omx