drm/i915/hdcp: Update CP as per the kernel internal state

Content Protection property should be updated as per the kernel
internal state. Let's say if Content protection is disabled
by userspace, CP property should be set to UNDESIRED so that
reauthentication will not happen until userspace request it again,
but when kernel disables the HDCP due to any DDI disabling sequences
like modeset/DPMS operation, kernel should set the property to
DESIRED, so that when opportunity arises, kernel will start the
HDCP authentication on its own.

Somewhere in the line, state machine to set content protection to
DESIRED from kernel was broken and IGT coverage was missing for it.
This patch fixes it.

v2:
- Fixing hdcp CP state in connector atomic check function
  intel_hdcp_atomic_check(). [Maarten]
  This will require to check hdcp->value in intel_hdcp_update_pipe()
  in order to avoid enabling hdcp, if it was already enabled.

v3:
- Rebased.

Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/350962/?series=72664&rev=2 #v1
Link: https://patchwork.freedesktop.org/patch/359396/?series=72251&rev=3 #v2
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630082048.22308-1-anshuman.gupta@intel.com