drm/i915: Nuke useless .set_signal_levels() wrappers

Now that .set_signal_levels() is used for HDMI as well, we can
remove the extra level of indirection and just plug the correct
stuff straight into .set_signal_levels().

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/20211001130107.1746-5-ville.syrjala@linux.intel.com