drm/i915: export error state to string conversion

In preparation for accessing error state from sysfs, export
error state to string conversion function. Also tuck buffer
error handling inside the function.

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>