[AMDGPU] Use default member initializers in Subtarget classes

Use default member initializers in AMDGPUSubtarget and subclasses. This
is to guard against adding a new feature boolean in AMDGPUSubtarget.h
but forgetting to initialize it to false in AMDGPUSubtarget.cpp.

This was mostly autogenerated by:
clang-tidy -checks=-*,cppcoreguidelines-prefer-member-initializer,modernize-use-default-member-init -header-filter=Subtarget -fix lib/Target/AMDGPU/*Subtarget.cpp

Differential Revision: https://reviews.llvm.org/D123613