drm/i915: there's no DSPPOS register on gen4+

So don't read it when capturing the error state. This solves some
"unclaimed register" messages on Haswell when we hang the GPU.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>