drm/i915/hdmi: remove unused intel_hdmi_hdcp2_protocol()

Unused, hiding from the compiler warnings behind the inline keyword.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200420140438.14672-1-jani.nikula@intel.com