drm/ttm: fix access to uninitialized variable.

The resource is not allocated yet, so no chance that this will work.

Use the placement instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210607171152.15914-1-christian.koenig@amd.com