drm/i915: debugfs: HDCP2.2 capability read

Adding the HDCP2.2 capability of HDCP src and sink info into debugfs
entry "i915_hdcp_sink_capability"

This helps the userspace tests to skip the HDCP2.2 test on non HDCP2.2
sinks.

v2:
  Rebased.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190507162745.25600-3-ramalingam.c@intel.com