hwc/overlay: Add support for pipe priorities, 1 pipe config for FB

If using source split to stage 2 pipes on the same mixer stage,
the left pipe needs to be a higher priority than the right.

Add API in overlay to compare pipe priorities and use this in
source split config to stage pipes accordingly.

Add support for 1 pipe config for FB if updating rect is within 2048
pixels.

Change-Id: I8b33d5ebd0f8765ee842bab128d8abd67a110145
diff --git a/liboverlay/overlay.h b/liboverlay/overlay.h
index d8615cd..44193be 100644
--- a/liboverlay/overlay.h
+++ b/liboverlay/overlay.h
@@ -114,6 +114,12 @@
      * displays
      */
     bool isPipeTypeAttached(utils::eMdpPipeType type);
+    /* Compare pipe priorities and return
+     * 1 if 1st pipe has a higher priority
+     * 0 if both have the same priority
+     *-1 if 2nd pipe has a higher priority
+     */
+    int comparePipePriority(utils::eDest pipe1Index, utils::eDest pipe2Index);
     /* Returns pipe dump. Expects a NULL terminated buffer of big enough size
      * to populate.
      */