drm/i915: WARN if HDCP signalling is enabled upon disable

HDCP signalling should not be left on, WARN if it is

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191212190230.188505-4-sean@poorly.run #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20200117193103.156821-4-sean@poorly.run #v3
Link: https://patchwork.freedesktop.org/patch/msgid/20200218220242.107265-4-sean@poorly.run #v4
Link: https://patchwork.freedesktop.org/patch/msgid/20200305201236.152307-4-sean@poorly.run #v5
Link: https://patchwork.freedesktop.org/patch/msgid/20200429195502.39919-4-sean@poorly.run #v6
Link: https://patchwork.freedesktop.org/patch/msgid/20200623155907.22961-4-sean@poorly.run #v7

Changes in v2:
-Added to the set in lieu of just clearing the bit
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:
-Rebased, variable name changed from 'ctl' to 'val'
Changes in v8:
-None
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200818153910.27894-4-sean@poorly.run

drivers/gpu/drm/i915/display/intel_ddi.c

@@ -1821,6 +1821,8 @@ void intel_ddi_disable_transcoder_func(const struct intel_crtc_state *crtc_state
 
 	ctl = intel_de_read(dev_priv, TRANS_DDI_FUNC_CTL(cpu_transcoder));
 
+	drm_WARN_ON(crtc->base.dev, ctl & TRANS_DDI_HDCP_SIGNALLING);
+
 	ctl &= ~TRANS_DDI_FUNC_ENABLE;
 
 	if (IS_GEN_RANGE(dev_priv, 8, 10))