drm/i915: remove LP_RING&friends from modeset code

The LP refers to 'low priority' as opposed to the high priority
ring on gen2/3. So lets constrain its use to the code of that era.

Unfortunately we can't yet completely remove the associated
macros from common headers and shove them into i915_dma.c to
the other dri1 legacy support code, a few cleanups are still
missing for that.

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