drm/i915: split out plane update code

Updating the planes is device specific, so create a new display callback
and use it in pipe_set_base.  (In fact we could go even further, valid
display plane bits have changed with each generation, as has tiled
buffer handling.)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>