drm/i915/gt: Do not attempt to reprogram IA/ring frequencies for dgfx

For dgfx, we do not need to reconfigure the IA/ring frequencies of the
main processors as they are distinct devices.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200219130119.1457693-1-chris@chris-wilson.co.uk