drm/i915: Don't check for NULL before freeing state

This is not necessary after the below commit.

commit a0211bb4
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Mon Mar 30 14:05:43 2015 +0300

    drm/atomic: Don't try to free a NULL state

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>