drm/i915: introduce i915_error_state_buf_init

Make function for struct i915_error_state_buf initialization
and export it, for sysfs and debugfs.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>