drm/i915: Fix a use-after-free in intel_execlists_retire_requests

Remove request from list before unreferencing it, in case it's actually
the only reference. (Found by Tvrtko Ursulin)

This issue has been most likely introduced in

commit 6d3d8274
Author: Nick Hoath <nicholas.hoath@intel.com>
Date:   Thu Jan 15 13:10:39 2015 +0000

    drm/i915: Subsume intel_ctx_submit_request in to drm_i915_gem_request

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>