v4l2videoenc: fix buffer allocator detect

It is just a kernel bug, I have tested in kernel 3.5.
gst-launch-1.0 --gst-debug=v4l2videoenc:9,v4l2allocator:9,v4l2:9 \
--gst-debug-no-color videotestsrc num-buffers=500 ! video/x-raw, \
format=\(string\)NV12,width=320,height=240 ! videoconvert \
! v4l2video9enc ! filesink location=1.h264
Could output color data.
480x320 works too.
640x480 will have the green color at the button part.
But the 1920x1080 is full green.