ENGR00119034 V4L2 overlay:Use DP to do CSC for preview on DPFG

Use DP to do CSC for preview on DPFG instead of using IC channel
so that we can get better performance. This change can make us
get rid of IC channel bandwidth limitation when using IC channel
to do upsizing from low resolution to high resolution and CSC.
This change doesn't touch V4L2 overlay for DPBG, because using DP
to do CSC will change the pixel format for DPBG(usually for GUI).
This change doesn't touch V4L2 capture, because users rarely do
upsizing with high multiple in this case.

Signed-off-by: Liu Ying <b17645@freescale.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>