HDCP1.4 key load process varies between Intel platform to platform.

For Gen9 platforms except BXT and GLK, HDCP1.4 key is loaded using
the GT Driver Mailbox interface. So all GEN9_BC platforms will use
the GT Driver Mailbox interface for HDCP1.4 key load.

v2:
  Using the IS_GEN9_BC for filtering the platforms [Ville]
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1544010283-20223-2-git-send-email-ramalingam.c@intel.com