drm/i915: there's no DSPADDR register on Haswell

So don't read it when we hang the GPU. This solves "unclaimed
register" messages.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Future-proof by adding a gen >= 7 check in addition to the
!IS_HSW check from Paulo's original patch, suggested by Ben.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>