drm: sti: rework init sequence

Use drm_dev_alloc() and drm_dev_register() instead of .load()
To simplify init sequence only create fbdev when requested
in output_poll_changed().

version 2:
remove call to drm_connector_unregister_all() and
drm_dev_set_unique()

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/1466514580-15194-4-git-send-email-benjamin.gaignard@linaro.org