drm/i915: Use variable for debugfs device status

Use a local variable to find SSEU runtime information
in various debugfs functions.

v2: Remove extra line breaks per feedback from Chris

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-2-stuart.summers@intel.com