drm/i915: Introduce .set_signal_levels() vfunc

Sort out some of the mess between intel_ddi.c intel_dp.c by
introducing a .set_signal_levels() vfunc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200420200610.31798-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>