media: imx-jpeg: use NV12M to represent non contiguous NV12
V4L2_PIX_FMT_NV12 requires num_planes equals to 1,
V4L2_PIX_FMT_NV12M requires num_planes equals to 2.
and mxc-jpeg supports 2 planes for nv12,
so we should use 4L2_PIX_FMT_NV12M instead of V4L2_PIX_FMT_NV12,
otherwise it will confuses gstreamer and prevent encoding and decoding.