drm/xe/gsc: Trigger a driver flr to cleanup the GSC on unload

GSC is only killed by an FLR, so we need to trigger one on unload to
make sure we stop it. This is because we assign a chunk of memory to
the GSC as part of the FW load, so we need to make sure it stops
using it when we release it to the system on driver unload. Note that
this is not a problem of the unload per-se, because the GSC will not
touch that memory unless there are requests for it coming from the
driver; therefore, no accesses will happen while Xe is not loaded,
but if we re-load the driver then the GSC might wake up and try to
access that old memory location again.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>