hwc: increase gscaler ring buffer size to 3

Fixes tearing reported in bug 7147037

Change-Id: I1b646b31fb23d1386c0cf48e2bfbfd3f1acc9378
Signed-off-by: Greg Hackmann <ghackmann@google.com>
diff --git a/libhwc/hwc.cpp b/libhwc/hwc.cpp
index 1281265..caaa2d5 100644
--- a/libhwc/hwc.cpp
+++ b/libhwc/hwc.cpp
@@ -86,7 +86,7 @@
     pthread_cond_t                  completion;
 };
 
-const size_t NUM_GSC_DST_BUFS = 2;
+const size_t NUM_GSC_DST_BUFS = 3;
 struct exynos5_gsc_data_t {
     void            *gsc;
     exynos_gsc_img  src_cfg;