drm/i915: Split intel_ddi_compute_min_voltage_level() into platform variants

The mess inside intel_ddi_compute_min_voltage_level() is illegible.
Clean it up a bit by splitting the internals into per-platform
functions.

TODO: make it a vfunc?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231128115138.13238-7-ville.syrjala@linux.intel.com


Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>