disp: msm: sde: fix check for uneven split with dest-scaler

Fix the check for uneven split across LMs when destination
scaler is used and reject the use-case since this is not
supported by the driver. This can happen if an odd-value
for the horizontal width is being split across 2 mixers.

The existing check was broken since it uses pass-by-value
with a zero initialized value that callee tries to update
while iterating over the DS configurations. Use pass-by-
reference with the previous DS configuration, if available,
instead of just 0 values so the proper comparisons can take
place.

Change-Id: I529faf57e43fb471b28b4d222260ea4d37217d4d
Signed-off-by: Steve Cohen <cohens@codeaurora.org>