hwc: Fix left half of fb for high res.

Left half of High res panel is not 2048 wide always.
When back-calculating for left mixer, use half of panel width.

Change-Id: Ifa2acd533f00cfef92a12d402e78b3491281a323
diff --git a/libhwcomposer/hwc_fbupdate.cpp b/libhwcomposer/hwc_fbupdate.cpp
index bfa33ca..7b34df9 100644
--- a/libhwcomposer/hwc_fbupdate.cpp
+++ b/libhwcomposer/hwc_fbupdate.cpp
@@ -231,7 +231,8 @@
         const int halfWidth = (displayFrame.right - displayFrame.left) / 2;
         const int height = displayFrame.bottom - displayFrame.top;
 
-        ovutils::Dim dposL(MAX_DISPLAY_DIM - halfWidth,
+        const int halfDpy = ctx->dpyAttr[mDpy].xres / 2;
+        ovutils::Dim dposL(halfDpy - halfWidth,
                            displayFrame.top,
                            halfWidth,
                            height);