drm/i915/gt: Wean workaround selftests off GEM context

The workarounds are tied to the GT and we should derive the tests local
to the GT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201210080240.24529-2-chris@chris-wilson.co.uk