drm/i915: De-wrapper bxt_ddi_phy_set_signal_levels()

Convert bxt_ddi_phy_set_signal_levels() to act as the full
.set_signal_levels() hook instead of going through a pointless wrapper.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-6-ville.syrjala@linux.intel.com