drm/i915: fix per-pipe reads after "cleanup"

In a few places I replaced reads of per-pipe registers with the actual
register offsets themselves (converting I915_READ(reg) to _PIPE(reg)).
Alexey caught this on his 9xx machine because the cursor control write
was affected.  A quick audit showed a few more places where I'd borked
a read, so here's a patch to fix things up.

Reported-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: compilation fix]
Tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>