Restore overlay support for RGB565 layers

This was accidentally removed by change-id
  I2b0eb35344631d162b30dd22ee0ad7651f22e6fa
which removed HAL formats 5551 and 4444.

Bug: 10748683
Change-Id: Id610083732f0495a7c4a29660c7c2c8750929510
diff --git a/libhwc/hwc.cpp b/libhwc/hwc.cpp
index 0e1477f..d5a7603 100644
--- a/libhwc/hwc.cpp
+++ b/libhwc/hwc.cpp
@@ -274,6 +274,8 @@
         return S3C_FB_PIXEL_FORMAT_RGBA_8888;
     case HAL_PIXEL_FORMAT_RGBX_8888:
         return S3C_FB_PIXEL_FORMAT_RGBX_8888;
+    case HAL_PIXEL_FORMAT_RGB_565:
+        return S3C_FB_PIXEL_FORMAT_RGB_565;
     case HAL_PIXEL_FORMAT_BGRA_8888:
         return S3C_FB_PIXEL_FORMAT_BGRA_8888;
     default: