vaapipostproc: Fix wrong selection of passthrough mode.

vaapipostproc: Fix wrong selection of passthrough mode.

The Current code path is falling back to passthorugh mode if there is no
vpp property set by the user explictily. But we should not use the
passthrough mode if the negotiated src pad caps have a differnt color space
format than sink pad caps (Even though the user didn't set the format property
explicitly).

https://bugzilla.gnome.org/show_bug.cgi?id=748184

Signed-off-by: Sreerenj Balachandran <sreerenj.balachandran@intel.com>