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

drivers/gpu/drm/i915/display/intel_ddi.c

@@ -3615,10 +3615,11 @@ static void intel_ddi_pre_enable_dp(struct intel_encoder *encoder,
 	/* MST will call a setting of MSA after an allocating of Virtual Channel
 	 * from MST encoder pre_enable callback.
 	 */
-	if (!intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST))
+	if (!intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST)) {
 		intel_ddi_set_dp_msa(crtc_state, conn_state);
 
 		intel_dp_set_m_n(crtc_state, M1_N1);
+	}
 }
 
 static void intel_ddi_pre_enable_hdmi(struct intel_encoder *encoder,

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -480,6 +480,8 @@ static void intel_mst_pre_enable_dp(struct intel_encoder *encoder,
 		intel_ddi_enable_pipe_clock(pipe_config);
 
 	intel_ddi_set_dp_msa(pipe_config, conn_state);
+
+	intel_dp_set_m_n(pipe_config, M1_N1);
 }
 
 static void intel_mst_enable_dp(struct intel_encoder *encoder,