drm/amd/display: add HDCP caps debugfs

Add debugfs to get HDCP capability. This is also useful for
kms_content_protection igt test.

Use:
	cat /sys/kernel/debug/dri/0/DP-1/hdcp_sink_capability
	cat /sys/kernel/debug/dri/0/HDMI-A-1/hdcp_sink_capability

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>