drm/i915: move error state to own compilation unit

Move error state generation and stringification to it's
own compilation unit. Sysfs also uses this so it can't be
under CONFIG_DEBUG_FS

This fixes a regression introduced in

commit ef86ddce


Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Thu Jun 6 17:38:54 2013 +0300

    drm/i915: add error_state sysfs entry

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66814
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>