VEXPRESS_CONFIG needs to either be missing, built-in, or modular when
pl111 is modular. Update the Kconfig to reflect the need.

Fixes: 4dc7c97d ("drm/pl111: depend on CONFIG_VEXPRESS_CONFIG")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210604014055.4060521-1-keescook@chromium.org