Otherwise s->input_format is always yuv420p.

This fixes invalid output format for hwframe download in the command
below:
./ffmpeg -init_hw_device vulkan -f lavfi \
	-i testsrc=duration=1,format=nv12 \
	-vf 'hwupload,format=vulkan,scale_vulkan=1024:768,hwdownload,format=nv12' \
	-f null -
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>