ENGR00120370 v4l2 output: fix display fail for blank fb during video play

1. Blank BG during video play on FG:
./mxc_v4l2_output.out -iw 320 -ih 240 -ow 1024 -oh 768 -d 3 qvga.yuv
echo 1 > /sys/class/graphics/fb0/blank
echo 0 > /sys/class/graphics/fb0/blank

2. The same input and output (ic_bypass):
./mxc_v4l2_output.out -iw 320 -ih 240 -ow 320 -oh 240 -d 3 qvga.yuv
echo 1 > /sys/class/graphics/fb0/blank
echo 0 > /sys/class/graphics/fb0/blank

Signed-off-by: Jason Chen <b02280@freescale.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>