msm: mdss: Do not override ARGC setting on LM

Layer mixer configuration overrides the ARGC
setting when it updates the blend color settings.
This removes the ARGC setting configured by post
processing module and calibration tool in tuning mode.
This change reads the ARGC setting and applies
it back during blend color setting.

Change-Id: I72612ad428e58a1e343e94ab965137628bcce661
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>