msm: camera: isp: Update packer format for NV21

NV21 format has odd and even bytes swapped compared to NV12. Updated
the packer format to configure the WM correctly for NV21.

Change-Id: I9c210b119a7878f4a62a7dacae68f9e28bb07d63
Signed-off-by: Harsh Shah <harshs@codeaurora.org>