amfenc: Retain a reference to D3D frames used as input during the encoding process

This fixes frame corruption issue when decoder started reusing frames
while they are still in use of encoding process

Issue with frame corruption  was reproduced using:

    avconv.exe -y -hwaccel d3d11va -hwaccel_output_format d3d11 -i input.h264 -an -c:v h264_amf output.mkv

It is recommended to use -extra_hw_frames 16 option in case if hw frames
number in pool is not enough

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>