This patch adds some protection against connectors being destroyed
before the HDCP workers are finished.

For check_work, we do a synchronous cancel after the connector is
unregistered which will ensure that it is finished before destruction.

In the case of prop_work, we can't do a synchronous wait since it needs
to take connection_mutex which could cause deadlock. Instead, we'll take
a reference on the connector when scheduling prop_work and give it up
once we're done.
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191212190230.188505-8-sean@poorly.run #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20200117193103.156821-8-sean@poorly.run #v3
Link: https://patchwork.freedesktop.org/patch/msgid/20200218220242.107265-8-sean@poorly.run #v4
Link: https://patchwork.freedesktop.org/patch/msgid/20200305201236.152307-8-sean@poorly.run #v5
Link: https://patchwork.freedesktop.org/patch/msgid/20200429195502.39919-8-sean@poorly.run #v6
Link: https://patchwork.freedesktop.org/patch/msgid/20200623155907.22961-8-sean@poorly.run #v7

Changes in v2:
-Added to the set
Changes in v3:
-Change the WARN_ON condition in intel_hdcp_cleanup to allow for
 initializing connectors as well
Changes in v4:
-None
Changes in v5:
-Change WARN_ON to drm_WARN_ON
Changes in v6:
-None
Changes in v7:
-None
Changes in v8:
-None
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200818153910.27894-8-sean@poorly.run