msm: mdss: ensure proper dynamic refresh programming for dual DSI

The DYNAMIC_REFRESH_CTRL register needs to be programmed for the
second DSI controller only for split DSI cases. This change fixes
an issue where the register programming is done for dual DSI cases
by adding a valid split DSI check.

Change-Id: I063f126eff19eb6a18e1e2b9f698b50bc94903f1
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>