drm/i915: Stop encryption for repeater with no sink

If a HDCP repeater is detected with zero downstream devices,
HDCP spec approves either of below actions:

1. Dont continue on second stage authentication. Disable encryption.
2. Continue with second stage authentication excluding the KSV list and
   on success, continue encryption.

Since disable encryption is agreed, repeater is not expected to have its
own display. So there is no consumption of the display content in such
setup.

Hence, incase of repeater with zero device count, this patch fails the
HDCP authentication and stops the HDCP encryption.

v2:
  Rephrased commit msg and added comments in code [Seanpaul]

v3:
  No changes. Added Reviewed-by tag.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1517609350-10698-3-git-send-email-ramalingam.c@intel.com