drm/i915: Show cursor status in debugfs/i915_display_info

I have the occasional absent cursor on i845 and I want to know why.
This should help by revealing the last known cursor state.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>