drm: Make helper vtable pointers type-safe

Originally the idea behind void* was to allow different sets of
helpers. But now we have that (with probe, plane, crtc and atomic
helpers) and we still just use the same set of vtables. That's the
only way to make the individual helpers modular and allow drivers to
pick&choose and transition between them. So this flexibility isn't
really needed. Also we have lots of non-vtable data meanwhile in core
structures too, this is not the first one at all.

Given that the void * is only trouble since gcc can't warn you if you
mix them up. Let's fix that and make them typesafe.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-5-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>