drm/vc4: Fix memory leak of BO array in hang state

In the Linux kernel, the following vulnerability has been resolved:

drm/vc4: Fix memory leak of BO array in hang state

The hang state's BO array is allocated separately with kzalloc() in

vc4_save_hang_state() but never freed in vc4_free_hang_state(). Add the

missing kfree() for the BO array before freeing the hang state struct.