omxvideoenc: restore OMX default target-bitrate if requested by user

omxvideoenc: restore OMX default target-bitrate if requested by user

0xffffffff is the magic number in gst-omx meaning 'the default value
defined in OMX'. This works fine with OMX parameters which are only set
once when starting the component but not with configs which can be
changed while PLAYING.
Save the actual OMX default bitrate so we can restore it later if user
sets back 0xffffffff on the property.

Added GST_OMX_PROP_OMX_DEFAULT so we stop hardcoding magic numbers
everywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=794998