drm/amd/powerplay: fix segment fault issue in multi-display case.

BugLink: http://bugs.launchpad.net/bugs/1580526



commit f9e9c08e upstream.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>