drm/radeon: Call radeon_suspend_kms() in radeon_pci_shutdown() for Loongson64

On the Loongson64 platform used with Radeon GPU, shutdown or reboot failed
when console=tty is in the boot cmdline.

radeon_suspend_kms() puts the hw in the suspend state, especially set fb
state as FBINFO_STATE_SUSPENDED:

        if (fbcon) {
                console_lock();
                radeon_fbdev_set_suspend(rdev, 1);
                console_unlock();
        }

Then avoid to do any more fb operations in the related functions:

        if (p->state != FBINFO_STATE_RUNNING)
                return;

So call radeon_suspend_kms() in radeon_pci_shutdown() for Loongson64 to fix
this issue, it looks like some kind of workaround like powerpc.

Co-developed-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org