drm/i915/dg2: Add vswing programming for SNPS phys

Vswing programming for SNPS PHYs is just a single step -- look up the
value that corresponds to the voltage level from a table and program it
into the SNPS_PHY_TX_EQ register.

Bspec: 53920
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723174239.1551352-26-matthew.d.roper@intel.com