drm/i915/tgl: Add gam instdone

This has been asked from us already. Prepare for the next
time.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029163841.5224-2-mika.kuoppala@linux.intel.com