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/i915_drv.h

@@ -1156,6 +1156,7 @@ struct i915_psr {
 	bool link_standby;
 	bool y_cord_support;
 	bool colorimetry_support;
+	bool alpm;
 };
 
 enum intel_pch {