disp: msm: sde: fix index variable used in CP pu check phase

Currently driver uses wrong index to retrieve partial update
ROI during check phase. This can lead to ROI being 0 for some
of the CP properties.

Change-Id: I885132e8d29eaf1353ed079eb283f7ca3a3d63a6
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>