drm/i915/dp: Move intel_dp_set_signal_levels() to intel_dp_link_training.c

intel_dp_set_signal_levels() is needed for link training, so move it to
intel_dp_link_training.c.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201229172201.4155327-1-imre.deak@intel.com