amdkfd: Copy from the proper user command pointer

8f1d57c1 ("amdkfd: don't open-code memdup_user()") mistakenly uses
an uninitialized local pointer, gcc complains:

  drivers/gpu/drm/amd/amdkfd/kfd_chardev.c: In function ‘kfd_ioctl_dbg_address_watch’:
  drivers/gpu/drm/amd/amdkfd/kfd_chardev.c:562:12: warning: ‘args_buff’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    args_buff = memdup_user(args_buff,
                ^

Fix it.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>