[NFC][AMDGPU] Default initialize the Subtarget

This is to address a static analizer warning:

The pointer field will point to an arbitrary memory location, any
attempt to write may cause corruption. In <unnamed>
R600DAGToDAGISel.:R600DAGToDAGISel (llvm::TargetMachine &,
livm::CodeGenOpt::Level): A pointer field is not initialized in the
constructor (CWE-457)

Differential Revision: https://reviews.llvm.org/D154414