drm/i915: rework legacy GFX HWS handling

To get the fun stuff out of the way, the legacy hws is allocated by
userspace when the gpu needs a gfx hws. And there's no reference-counting
going on, so userspace can simply screw everyone over.

At least it's not as horrible as i810, where the ringbuffer is allocated
by userspace ...

We can't fix this disaster, but we can at least tidy up the code a
bit to make things clearer:
- Drop the drm ioremap indirection.
- Add a new new read_legacy_status_page to paper over the differences
  between the legacy gfx hws and the physical hws shared with the
  new ringbuffer code.
- Add a pointer in dev_priv->dri1 for the cpu addresses - that one is
  an iomem remapping as opposed to all other hw status pages. This is
  just prep work to make sparse happy.

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>