This patch is required for HDCP over MST. If a port is being used for
multiple HDCP streams, we don't want to fully disable HDCP on a port if
one of them is disabled. Instead, we just disable the HDCP signalling on
that particular pipe and exit early. The last pipe to disable HDCP will
also bring down HDCP on the port.

In order to achieve this, we need to keep a refcount in intel_digital_port
and protect it using a new hdcp_mutex.

Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203173638.94919-8-sean@poorly.run #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20191212190230.188505-9-sean@poorly.run #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20200117193103.156821-9-sean@poorly.run #v3
Link: https://patchwork.freedesktop.org/patch/msgid/20200218220242.107265-9-sean@poorly.run #v4
Link: https://patchwork.freedesktop.org/patch/msgid/20200305201236.152307-9-sean@poorly.run #v5
Link: https://patchwork.freedesktop.org/patch/msgid/20200429195502.39919-9-sean@poorly.run #v6
Link: https://patchwork.freedesktop.org/patch/msgid/20200623155907.22961-10-sean@poorly.run #v7

Changes in v2:
-Move the toggle_signalling call into _intel_hdcp_disable so it's called from check_work
Changes in v3:
-None
Changes in v4:
-None
Changes in v5:
-Change WARN_ON to drm_WARN_ON
Changes in v6:
-None
Changes in v7:
-Split minor intel_hdcp_disable refactor into separate patch (Ramalingam)
Changes in v8:
-None
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200818153910.27894-10-sean@poorly.run