drm: stm: remove "default y" in Kconfig

To do not force stm driver to be build by default

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1499699248-25776-1-git-send-email-benjamin.gaignard@linaro.org