hwc: Set GPU performance hint.

1. Set the GPU hint flag to high for MIXED/GPU composition only for
   first frame after MDP to GPU/MIXED mode transition.
2. Set the GPU hint to default if the current composition type is GPU
   due to idle fallback or MDP composition.

Change-Id: I208a778017435a5a4620142da9d9fb3c50e09155
diff --git a/libhwcomposer/hwc.cpp b/libhwcomposer/hwc.cpp
index 893fee0..bfcfa78 100644
--- a/libhwcomposer/hwc.cpp
+++ b/libhwcomposer/hwc.cpp
@@ -212,6 +212,7 @@
             if(ctx->mCopyBit[dpy])
                 ctx->mCopyBit[dpy]->prepare(ctx, list, dpy);
         }
+        setGPUHint(ctx, list);
     }
     return 0;
 }