hwc: Reset ROI for all the prepare calls.

Partial update ROI needs to be reset at the beginning of every
frame even when we don't have any application layers.

Change-Id: Ib4b604dfa9b589e59150ecc66f48a396f0b37af5
diff --git a/libhwcomposer/hwc.cpp b/libhwcomposer/hwc.cpp
index 267815f..1a302d6 100644
--- a/libhwcomposer/hwc.cpp
+++ b/libhwcomposer/hwc.cpp
@@ -347,6 +347,7 @@
     for (int32_t i = ((int32_t)numDisplays-1); i >=0 ; i--) {
         hwc_display_contents_1_t *list = displays[i];
         int dpy = getDpyforExternalDisplay(ctx, i);
+        resetROI(ctx, dpy);
         switch(dpy) {
             case HWC_DISPLAY_PRIMARY:
                 ret = hwc_prepare_primary(dev, list);