msm: mdss: fix data path for concurrent writeback

- Read data-path flag set by client for CWB, and
populate CWB output layer flags.
- Fix data path register setting for right mixer

Change-Id: I3599ce1b19df48c72a4c951be76b813704b8a124
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>