drm/i915: Export the generic, not arch specific, intel_update_watermarks()

Rather than export every single architecture specific update_wm, just
export the wrapper around the display vtable.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>