drm/i915/mst: Drop pre_pll_enable null check

MST is only supported in DDI ports that have this hook, so the null
check can be dropped.
Suggested-by: Imre Deak <imre.deak@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107235449.32264-3-jose.souza@intel.com

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -209,8 +209,7 @@ static void intel_mst_pre_pll_enable_dp(struct intel_encoder *encoder,
 	struct intel_digital_port *intel_dig_port = intel_mst->primary;
 	struct intel_dp *intel_dp = &intel_dig_port->dp;
 
-	if (intel_dp->active_mst_links == 0 &&
-	    intel_dig_port->base.pre_pll_enable)
+	if (intel_dp->active_mst_links == 0)
 		intel_dig_port->base.pre_pll_enable(&intel_dig_port->base,
 						    pipe_config, NULL);
 }