drm/i915: Remove pointer indirection for hangcheck_state local

Use the local on-stack struct directly rather than hide it behind a
pointer. This should be both clearer for the reader and the compiler (we
rely on the compiler seeing through the functions to spot uninitialized
uses of the local).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171219130948.6282-1-chris@chris-wilson.co.uk