kgdb: Make "kgdbcon" work properly with "kgdb_earlycon"

On my system the kernel processes the "kgdb_earlycon" parameter before
the "kgdbcon" parameter.  When we setup "kgdb_earlycon" we'll end up
in kgdb_register_callbacks() and "kgdb_use_con" won't have been set
yet so we'll never get around to starting "kgdbcon".  Let's remedy
this by detecting that the IO module was already registered when
setting "kgdb_use_con" and registering the console then.

As part of this, to avoid pre-declaring things, move the handling of
the "kgdbcon" further down in the file.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200630151422.1.I4aa062751ff5e281f5116655c976dff545c09a46@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>