drm/radeon: fix write back suspend regression with uvd v2

UVD ring can't use scratch thus it does need writeback buffer to keep
a valid address or radeon_ring_backup will trigger a kernel fault.

It's ok to not unpin the write back buffer on suspend as it leave in
gtt and thus does not need eviction.

v2: Fix the uvd case.

Reported and tracked by Wojtek <wojtask9@wp.pl>

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_uvd.c b/drivers/gpu/drm/radeon/radeon_uvd.c
index 906e5c0..9f55ade 100644
--- a/drivers/gpu/drm/radeon/radeon_uvd.c
+++ b/drivers/gpu/drm/radeon/radeon_uvd.c
@@ -159,7 +159,17 @@
 	if (!r) {
 		radeon_bo_kunmap(rdev->uvd.vcpu_bo);
 		radeon_bo_unpin(rdev->uvd.vcpu_bo);
+		rdev->uvd.cpu_addr = NULL;
+		if (!radeon_bo_pin(rdev->uvd.vcpu_bo, RADEON_GEM_DOMAIN_CPU, NULL)) {
+			radeon_bo_kmap(rdev->uvd.vcpu_bo, &rdev->uvd.cpu_addr);
+		}
 		radeon_bo_unreserve(rdev->uvd.vcpu_bo);
+
+		if (rdev->uvd.cpu_addr) {
+			radeon_fence_driver_start_ring(rdev, R600_RING_TYPE_UVD_INDEX);
+		} else {
+			rdev->fence_drv[R600_RING_TYPE_UVD_INDEX].cpu_addr = NULL;
+		}
 	}
 	return r;
 }
@@ -178,6 +188,10 @@
 		return r;
 	}
 
+	/* Have been pin in cpu unmap unpin */
+	radeon_bo_kunmap(rdev->uvd.vcpu_bo);
+	radeon_bo_unpin(rdev->uvd.vcpu_bo);
+
 	r = radeon_bo_pin(rdev->uvd.vcpu_bo, RADEON_GEM_DOMAIN_VRAM,
 			  &rdev->uvd.gpu_addr);
 	if (r) {