drm/exynos: balance framebuffer refcount

exynos_drm_crtc_mode_set assigns primary framebuffer to plane without
taking reference. Then during framebuffer removal it is dereferenced twice,
causing oops. The patch fixes it.
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/exynos/exynos_drm_crtc.c

@@ -145,6 +145,7 @@ exynos_drm_crtc_mode_set(struct drm_crtc *crtc, struct drm_display_mode *mode,
 
 	plane->crtc = crtc;
 	plane->fb = crtc->primary->fb;
+	drm_framebuffer_reference(plane->fb);
 
 	return 0;
 }