msm: mdss: remove obsolete method of mixer register read/writes

Starting apq8084, mixer offsets have changed compared to previous
chip-sets. Some of the code is still using old method of reading
and writing to mixer registers which leads to incorrect hw programming.
Replace these mixer register read/writes with working method.

CRs-Fixed: 564273
Change-Id: I2aa1c5d24c6a1c5b89f77bd8d450c0831dd720b7
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>