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: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>