vaapidownload: fix src caps format error.
vaapidownload: fix src caps format error.

This fixes direct linking of vaapidownload element to xvimagesink with
VA drivers supporting vaGetImage() from the native VA surface format to
a different VA image format. i.e. color conversion during download.

http://bugzilla.gnome.org/show_bug.cgi?id=703937

Signed-off-by: Gwenole Beauchesne <729713629d9640cc095f1c77c692e1881f8093c4@intel.com>

GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi