Currently the default swscale flags for simple filter graph is bicubic,
however for complex filter graph it uses bilinear as decleared in scale
filter.

$ffmpeg -v verbose -i input.mp4 -vf format=yuv420p,scale=800x600 -an -f null -
[Parsed_scale_1 @ 0x7f86d2c160c0] w:1920 h:1080 fmt:yuv420p sar:0/1 -> w:800 h:600 fmt:yuv420p sar:0/1 flags:0x4

$ffmpeg -v verbose -i input.mp4 -filter_complex format=yuv420p,scale=800x600 -an -f null -
[Parsed_scale_1 @ 0x7f8779e046c0] w:1920 h:1080 fmt:yuv420p sar:0/1 -> w:800 h:600 fmt:yuv420p sar:0/1 flags:0x2

Use default swscale flags (bicubic currently) for scale filter.
- Remove flags="bilinear" from vf_scale
- Update the FATE refs
Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>