msm: mdss: Fix src_split mixer settings for 4k HDMI usecase

Change Ic07deab12aa43bc4f61762303ed367fe3131389f,
msm: mdss: enable source split with dual-dsi partial update,
introduced the problem of only half screen getting updated for
4k HMDI. Since HDMI does not have a slave controller, it was not
configuring the src_split_req flag correctly. 4k HDMI uses both
left and right mixers controlled by a single ctl. Fix the issue
by setting the src_split_req appropriately.

Change-Id: I333afd85ae5dc88610c352b557a834526bafcdfd
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>