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When crtc state need_modeset is true it is not necessary
it is going to be a real modeset, it can turns to be a
fastset instead of modeset.
This turns content protection property to be DESIRED and hdcp
update_pipe left with property to be in DESIRED state but
actual hdcp->value was ENABLED.

This issue is caught with DP MST setup, where we have multiple
connector in same DP_MST topology. When disabling HDCP on one of
DP MST connector leads to set the crtc state need_modeset to true
for all other crtc driving the other DP-MST topology connectors.
This turns up other DP MST connectors CP property to be DESIRED
despite the actual hdcp->value is ENABLED.
Above scenario fails the DP MST HDCP IGT test, disabling HDCP on
one MST stream should not cause to disable HDCP on another MST
stream on same DP MST topology.

v2:
- Fixed connector->base.registration_state == DRM_CONNECTOR_REGISTERED
  WARN_ON.

v3:
- Commit log improvement. [Uma]
- Added a comment before scheduling prop_work. [Uma]

Fixes: 33f9a623

 ("drm/i915/hdcp: Update CP as per the kernel internal state")
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Tested-by: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111081120.28417-2-anshuman.gupta@intel.com