drm: sti: make driver use devm_of_platform_populate()

This make sure that of_platform_depopulate() is called if an error
occur in probe after populating the date from the device tree.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1487952874-23635-3-git-send-email-benjamin.gaignard@linaro.org