drm: sti: use late_register and early_unregister callbacks

Make sti driver use register callback to move debugfs
initialization out of sub-components creation.
This will allow to convert driver .load() to
drm_dev_alloc() and drm_dev_register().

sti_compositor bring up 2 crtc but only one debugfs init is
needed so use drm_crtc_index to do it on the first one.
This can't be done in sti_drv because only sti_compositor have
access to the devices.
It is almost the same for sti_encoder which handle multiple
encoder while one only debugfs entry is needed so add a boolean
to avoid multiple debugfs initialization

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/1466514580-15194-3-git-send-email-benjamin.gaignard@linaro.org

drivers/gpu/drm/sti/sti_mixer.h

@@ -55,6 +55,8 @@ int sti_mixer_active_video_area(struct sti_mixer *mixer,
 
 void sti_mixer_set_background_status(struct sti_mixer *mixer, bool enable);
 
+int sti_mixer_debugfs_init(struct sti_mixer *mixer, struct drm_minor *minor);
+
 /* depth in Cross-bar control = z order */
 #define GAM_MIXER_NB_DEPTH_LEVEL 6