drm/i915: Clear the in-use marker on execbuf failure

If we fail to unbind the vma (due to a signal on an active buffer that
needs to be moved for the next execbuf), then we need to clear the
persistent tracking state we setup for this execbuf.

Fixes: c7c6e46f913b ("drm/i915: Convert execbuf to use struct-of-array packing for critical fields")
Testcase: igt/gem_fenced_exec_thrash/no-spare-fences-busy*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.14+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219140144.24004-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -505,6 +505,8 @@ eb_add_vma(struct i915_execbuffer *eb, unsigned int i, struct i915_vma *vma)
 		list_add_tail(&vma->exec_link, &eb->unbound);
 		if (drm_mm_node_allocated(&vma->node))
 			err = i915_vma_unbind(vma);
+		if (unlikely(err))
+			vma->exec_flags = NULL;
 	}
 	return err;
 }