drm/i915/psr: enable ALPM for psr2

As per edp1.4 spec , alpm is required for psr2 operation as it's
used for all psr2  main link power down management and alpm enable
bit must be set for psr2 operation.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: vathsala nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Patil Deepti <deepti.patil@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483356663-32668-6-git-send-email-vathsala.nagaraju@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -209,7 +209,11 @@ static void hsw_psr_enable_sink(struct intel_dp *intel_dp)
 		drm_dp_dpcd_writeb(&intel_dp->aux,
 				DP_SINK_DEVICE_AUX_FRAME_SYNC_CONF,
 				DP_AUX_FRAME_SYNC_ENABLE);
-
+	/* Enable ALPM at sink for psr2 */
+	if (dev_priv->psr.psr2_support && dev_priv->psr.alpm)
+		drm_dp_dpcd_writeb(&intel_dp->aux,
+				DP_RECEIVER_ALPM_CONFIG,
+				DP_ALPM_ENABLE);
 	if (dev_priv->psr.link_standby)
 		drm_dp_dpcd_writeb(&intel_dp->aux, DP_PSR_EN_CFG,
 				   DP_PSR_ENABLE | DP_PSR_MAIN_LINK_ACTIVE);