drm/sti: Build monolithic driver

There's no use building the individual drivers as separate modules
because they are all only useful if combined into a single DRM/KMS
device.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>