drm/i915/dg2: use existing mechanisms for SNPS PHY translations

We use encoder->get_buf_trans() in many places, for example
intel_ddi_dp_voltage_max(), and the hook was set to some old platform's
function for DG2 SNPS PHY. Convert SNPS PHY to use the same translation
mechanisms as everything else.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210813115151.19290-2-jani.nikula@intel.com