AMDGPU: Set flat_scratch from flat_scratch_init reg

This was hardcoded to the static private size, but this
would be missing the offset and additional size for someday
when we have dynamic sizing.

Also stops always initializing flat_scratch even when unused.

In the future we should stop emitting this unless flat instructions
are used to access private memory. For example this will initialize
it almost always on VI because flat is used for global access.

llvm-svn: 260658