drm: Nuke drm_framebuffer->helper_private

It's completely unused and there's really no reason for this:
- drm_framebuffer structures are invariant after creation, no need for
  helpers to manipulate them.
- drm_framebuffer structures should just be embedded (and that's what
  all the drivers do).

Stumbled over this since some folks are apparently concerned with the
overhead of struct drm_framebuffer and this is an easy 8 byte saving.

More could be gained by ditching the legacy fields and recomputing
stuff from the fourcc value. But that would require some drm-wide
cocci and real justification.

Cc: gary.k.smith@intel.com
Reviewed-by: NDavid Herrmann <dh.herrmann@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

include/drm/drm_crtc.h

@@ -210,8 +210,6 @@ struct drm_framebuffer {
 	int flags;
 	uint32_t pixel_format; /* fourcc format */
 	struct list_head filp_head;
-	/* if you are using the helper */
-	void *helper_private;
 };
 
 struct drm_property_blob {