msm: mdss: Fix mixer config for 8084 RGB3 & ViG3 pipes

Layer mixer configuration of RGB3 & ViG3 pipes of 8084 MDSS is different
than 0-2 pipes. Current implementation works only if layer passing through
these pipes is base layer. Fix this by implementing correct mixer config
for other blending stages.

Change-Id: I2410b3e601080a52706195a2507e78da457dc87a
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>