Going forward, struct intel_display shall replace struct
drm_i915_private as the main display device data pointer type. Convert
intel_hdmi.[ch] to struct intel_display. Remove intel_hdmi_to_i915().

Some stragglers are left behind where needed.
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/fa74b67935eb7e8084f57688a9683a36cb1d1a4c.1725012870.git.jani.nikula@intel.com