drm_hwcomposer: Switch to triple buffering

This increases the performance of the UI (30->40 fps).

BUG=none
TEST=boot to UI, swipe between desktops, measure fps

Change-Id: Ie0ff4b51c3db402a40c9676d3f09d48d85097bce
diff --git a/hwcomposer.cpp b/hwcomposer.cpp
index 6c007bc..5d395b9 100644
--- a/hwcomposer.cpp
+++ b/hwcomposer.cpp
@@ -43,7 +43,7 @@
 #include <ui/PixelFormat.h>
 
 #define UM_PER_INCH 25400
-#define HWC_FB_BUFFERS 2
+#define HWC_FB_BUFFERS 3
 
 namespace android {