drm/i915/hdmi: convert intel_hdmi_to_dev to intel_hdmi_to_i915

Prefer i915 over drm pointer.

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/20210921110244.8666-1-jani.nikula@intel.com