msm: mdss: fix split_mode setting during mode switch

During mode switch, split_mode was set to pp split by checking
if the panel is in split_mode and MDP supports pp split. This
leads to faulty configuration as pp split should be set only
when the panel setting has pp split and MDP support it. Add
proper checks to set the split mode correctly.

Change-Id: I2f2c10bebcb3ce7d38c37ff09a25d864a89e646a
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>