drm/i915/display: add I915 conditional build to g4x_hdmi.h

Add stubs for !I915.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ba2eea7ac081d0858a573062f197e00f3b186faf.1694514689.git.jani.nikula@intel.com