drm/radeon: avoid kernel segfault in vce when gpu fails to resume

When GPU fails to resume we can not trust that value we write to GPU
memory will post and we might get garbage (more like 0xffffffff on
x86) when reading them back. This trigger out of range memory access
in the kernel inside the vce resume code path.

This patch use canonical value to compute offset instead of reading
back value from GPU memory.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>