drm/i915/hdcp: move update pipe code to hdcp

The DDI encoder code shouln't have to know about the guts of
HDCP. Abstract the pipe update code to a new intel_hdcp_update_pipe() in
intel_hdcp.c. No functional changes.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128163803.5954-1-jani.nikula@intel.com