drm/i915: Enable DDI IO power domains in the DP MST path

Commit 62b69566 ("drm/i915: Only enable DDI IO power domains after
enabling DPLL") changed how the DDI IO power domains get enabled, but
neglected the need to enable those domains when enabling a DP connector
with MST enabled, leading to

    Kernel panic - not syncing: Timeout: Not all CPUs entered broadcast exception handler

Fixes: 62b69566 ("drm/i915: Only enable DDI IO power domains after enabling DPLL")
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301141318.3607-2-ander.conselvan.de.oliveira@intel.com

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -163,6 +163,9 @@ static void intel_mst_pre_enable_dp(struct intel_encoder *encoder,
 		intel_ddi_clk_select(&intel_dig_port->base,
 				     pipe_config->shared_dpll);
 
+		intel_display_power_get(dev_priv,
+					intel_dig_port->ddi_io_power_domain);
+
 		intel_prepare_dp_ddi_buffers(&intel_dig_port->base);
 		intel_dp_set_link_params(intel_dp,
 					 pipe_config->port_clock,