drm/i915/gsc: flush the GSC worker before wedging on unload

If we unload the driver and wedge before the GSC worker is complete,
the worker will hit an error on its submission to the GSC engine and
then exit. This is hard to hit for a user, but it is reproducible
with skipping selftests. The error is handled gracefully by the
worker, so there are no functional issues, but we still end up with
an error message in dmesg, which is something we want to avoid as
this is a supported scenario. We could modify the worker to better
handle a wedging occurring during its execution, but that gets
complicated for a couple of reasons:
- We do want the error on runtime wedging, because there are
  implications for subsystems outside of GT (i.e., PXP, HDCP), it's
  only the error on driver unload that we want to silence.
- The worker is responsible for multiple submissions (GSC FW load,
  HuC auth, SW proxy), so all of those will have to be adapted to
  handle the wedged_on_fini scenario.
Therefore, it's much simpler to just wait for the worker to be done
before wedging on driver removal, also considering that the worker
will likely already be idle in the great majority of non-selftest
scenarios.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223172120.3304293-2-daniele.ceraolospurio@intel.com