fix GPU drawing for 8 bit alpha bitmaps

BUG=

Review URL: https://codereview.appspot.com/7070052

git-svn-id: http://skia.googlecode.com/svn/trunk@7081 2bbb7eff-a529-9590-31e7-b0007b416f81

src/gpu/SkGpuDevice.cpp

@@ -1200,7 +1200,9 @@ void SkGpuDevice::drawBitmapCommon(const SkDraw& draw,
 
     GrPaint grPaint;
     SkAutoCachedTexture colorLutTexture;
-    if (!skPaint2GrPaintNoShader(this, paint, true, false, &colorLutTexture, &grPaint)) {
+    
+    bool alphaOnly = !(SkBitmap::kA8_Config == bitmap.config());
+    if (!skPaint2GrPaintNoShader(this, paint, alphaOnly, false, &colorLutTexture, &grPaint)) {
         return;
     }
     GrTextureParams params;