drm/sti: Use drm_fb_cma_fbdev_init/fini()

Use drm_fb_cma_fbdev_init() and drm_fb_cma_fbdev_fini() which relies on
the fact that drm_device holds a pointer to the drm_fb_helper structure.
This means that the driver doesn't have to keep track of that.
Also use the drm_fb_helper functions directly.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115142001.45358-13-noralf@tronnes.org

drivers/gpu/drm/sti/sti_drv.h

@@ -24,7 +24,6 @@ struct sti_private {
 	struct sti_compositor *compo;
 	struct drm_property *plane_zorder_property;
 	struct drm_device *drm_dev;
-	struct drm_fbdev_cma *fbdev;
 };
 
 extern struct platform_driver sti_tvout_driver;