Enable subdiv by default when set in the UsdGeomMesh schema

[sebastienblor]

Currently, even when the usd mesh attribute subdivisionScheme is set e.g. to catclark (which is the default), there is no subdivision in Arnold because we force the default value of subdiv_iterations to be equal to 0.
Therefore, there is no way to enable subdivision without setting the arnold specific attribute.
We should consider changing this and let arnold have a default subdiv iteration of 1

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15473