drm/i915: move intel_dp_set_m_n() to encoder for DDI platforms

intel_dp_set_m_n() has a clear place in the DDI DP specific pre-enable
hook.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128162850.8660-3-jani.nikula@intel.com