Fix AVG codegen

We weren't returning the right merge function for decimal in
GetAvgFunction(). Someday the functions will be registered in the FE
like for scalar functions.

Change-Id: I1153ef8570b0e78f0925b7d3d58ec3b0fbb2c589
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4336
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins