drm/i915: Fix KBL HDCP2.2 encrypt status signalling

HDCP transmitter is supposed to indicate the HDCP encryption status of
the link through enc_en signals in a window of time called "window of
opportunity" defined by HDCP HDMI spec.

But on KBL this timing of signalling has an issue. To fix the issue this
WA of resetting the signalling is required.

v2:
  WA is moved into the toggle_signalling [Daniel]
v3:
  Commit msg is rewritten with more information
v4:
  Reviewed-by Daniel.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1550338640-17470-17-git-send-email-ramalingam.c@intel.com