dts: sa8195: fix display can't work in DP_MST mode

KMS driver reported that "unable to find appropriate mixers"
when work in DP_MST mode, and there have two primary layer
mixers in dts. It can be fixed by to modify the secondary
layer mixer's preference to "none" to meet DP's requirement.

Change-Id: I3f1356bef7b2159ce3fcb6c151cd4ec906559008
Signed-off-by: Zhiqiang Liu <zhiqliu@codeaurora.org>