info: move category level threshold check into log function dispatcher
info: move category level threshold check into log function dispatcher

Minor optimisation: check category log level earlier in the
log function dispatcher and not only in the default log
function.

https://bugzilla.gnome.org/show_bug.cgi?id=745213

shelsonjava/gstreamer,mparis/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,jpxiong/gstreamer,mparis/gstreamer,krichter722/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer