drm/i915: Use the cpu_transcoder in intel_hdcp to toggle HDCP signalling

Instead of using intel_dig_port's encoder pipe to determine which
transcoder to toggle signalling on, use the cpu_transcoder field already
stored in intel_hdmi.

This is particularly important for MST.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.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/20191212190230.188505-6-sean@poorly.run #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20200117193103.156821-6-sean@poorly.run #v3
Link: https://patchwork.freedesktop.org/patch/msgid/20200218220242.107265-6-sean@poorly.run #v4
Link: https://patchwork.freedesktop.org/patch/msgid/20200305201236.152307-6-sean@poorly.run #v5
Link: https://patchwork.freedesktop.org/patch/msgid/20200429195502.39919-6-sean@poorly.run #v6
Link: https://patchwork.freedesktop.org/patch/msgid/20200623155907.22961-6-sean@poorly.run

 #v7

Changes in v2:
-Added to the set
Changes in v3:
-s/hdcp/hdmi/ in commit msg (Ram)
Changes in v4:
-Rebased on intel_de_(read|write) change
Changes in v5:
-Update hdcp->cpu_transcoder in intel_hdcp_enable so it works with pipe != 0
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/20200818153910.27894-6-sean@poorly.run