drm/i915: Fix GEN9 HDCP1.4 key load process

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