ENGR00291103 IPUv3 common:Don't initialize PRP_VF for MEM_VDI_MEM

ENGR00291103 IPUv3 common:Don't initialize PRP_VF for MEM_VDI_MEM

Since the channel MEM_VDI_MEM doesn't use the PRP_VF task, this
patch removes the code to initialize the PRP_VF task for the
channel MEM_VDI_MEM. This change may fix the issue caused by
the unnesessary PRP_VF task output resolution limitation check.
The issue can be reproduced by the following unit test case:

mxc_vpu_test.out -D "-f 2 -y 2 -v m -i
1080i_shields1088i2997_shields_ter_4x300_15fps_track1.h264"

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>