msm: mdss: avoid flickering by configuring both dsi ctrl at same time

At DFPS, to avoid flickering at right half panel, right dsi controller
needs to be configured along with left controller at same time when split
display enabled.

Change-Id: Ibbe451bd72e9740d5902d664a5e9e73db90bba5a
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>