drm/i915: dynamically set up the render ring functions and params

Our hw is simply not well-designed enough that it neatly fits into
boxes. Everywhere else we set up vtables and similar things
dynamically using switch statements - it's simply much more flexible.

This is prep work to rework the pre-gen6 ring irq stuff - it'll add a
few more differences. With the current const struct templates, that
would be a mess.

This leads to some unfortunate duplication with the old dri1 code, but
we can reap that again because gen6 isn't actually supported there.
But that's for a separate patch.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>