drm/i915/mst: Set intel_dp_set_m_n() for MST slaves

Commit 1c9d2eb2 ("drm/i915: move intel_dp_set_m_n() to encoder for
DDI platforms") moved the intel_dp_set_m_n() from hsw_crtc_enable()
to intel_ddi_pre_enable_dp() but it missed add it to
intel_mst_pre_enable_dp() causing MST slaves to not work.

v2: Not setting intel_ddi_set_dp_msa() twice for MST master

Fixes: 1c9d2eb2 ("drm/i915: move intel_dp_set_m_n() to encoder for DDI platforms")
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211185008.30806-1-jose.souza@intel.com