drm/i915: there's no PIPESTAT on HAS_PCH_SPLIT platforms

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

V2: Check for HAS_PCH_SPLIT instead of Gen5+ because VLV still has
this register.

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