drm/i915/gen9: Write dc state debugmask bits only once

DMC debugmask bits should stick so no need to write them
everytime dc state is changed.

v2: Write after firmware has been successfully loaded (Ville)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455808874-22089-5-git-send-email-mika.kuoppala@intel.com