drm/i915: Fix inconsistent indenting in i915_error_state_to_str()

smatch complains:
	drivers/gpu/drm/i915/i915_gpu_error.c:503 i915_error_state_to_str()
	warn: inconsistent indenting

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467470166-31717-4-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>