amdkfd: Copy from the proper user command pointer

BugLink: http://bugs.launchpad.net/bugs/1546572

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>
(cherry picked from commit 39c01bf9)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/amd/amdkfd/kfd_chardev.c

@@ -558,8 +558,7 @@ static int kfd_ioctl_dbg_address_watch(struct file *filep,
 		return -EINVAL;
 
 	/* this is the actual buffer to work with */
-
-	args_buff = memdup_user(args_buff,
+	args_buff = memdup_user(cmd_from_user,
 				args->buf_size_in_bytes - sizeof(*args));
 	if (IS_ERR(args_buff))
 		return PTR_ERR(args_buff);