drm/i915: Optimize HDCP key load

HDCP key need not be cleared on each hdcp disable. And HDCP key Load
is skipped if key is already loaded.

v2:
  No change. Added Reviewed-by tag.

v3:
  No change.

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/1517609350-10698-6-git-send-email-ramalingam.c@intel.com