msm: mdss: fix source split issues with base layer

When source split is enabled, all the layers residing on right
layer mixer (LM) have their dst_x offset relative to left LM's left top
co-ordinate except base layer. Base layer is a special case where it
doesn't abide by this HW rule and to make it work when source split is
enabled, either dst_x of base layer on right LM needs to be relative to
right LM's left top or base layer needs to be staged on both LMs. Since
later approach doesn't fit well with other use-cases, use the first
approach.

Change-Id: Ifa36cdf26f6baf68723d0e0ccad6cad089e8bedb
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>