drm/i915: Introduce g4x_hdmi_compute_config()

Start pulling some of the more platform specific things out from
intel_hdmi_compute_config(). has_pch_encoder is clearly one
such thing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107194604.15227-2-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>