Don't call finish on GrDrawTarget if we drew no batches

This might give us some minor perf gains in vulkan since we won't be submitting
command buffers that we've added to batch draws to. Running nanobench this does
seem to happen pretty often.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2179903008

Review-Url: https://codereview.chromium.org/2179903008

src/gpu/GrDrawTarget.cpp

@@ -216,6 +216,9 @@ void GrDrawTarget::prepareBatches(GrBatchFlushState* flushState) {
 }
 
 void GrDrawTarget::drawBatches(GrBatchFlushState* flushState) {
+    if (0 == fRecordedBatches.count()) {
+        return;
+    }
     // Draw all the generated geometry.
     SkRandom random;
     GrRenderTarget* currentRT = nullptr;