msm: mdss: fix split display configuration

Split display configuration requires that lower
and upper control registers get programmed accordingly.
This change fixes a configuration bit which can cause
page faults if the timing engine gets flushed after enabled.

Change-Id: I5b5112f36ee3855a81632eab495c94d96893bdfa
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>