msm: mdss: fix split_lm flag setting for HDMI usecases

HDMI 4k uses dual layer mixer controlled by a single controller,
but fails to set the split_lm flag appropriately. This leads to
false comparisons in few usecases. Set the split_lm flag, and
the associated structures appropriately for different HDMI
configurations.

Change-Id: I7a294bc0ad7a45b92b6aceebbd6efa9d5f09a8ca
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>