drm/ttm: clear the ttm_tt when bo->resource is NULL

drm/ttm: clear the ttm_tt when bo->resource is NULL

In the next few patches, when initially creating a ttm BO, the
bo->resource is NULL, and the driver is then expected to handle the
initial dummy move.  However, if this is created as a system resource
the first ttm_tt we create will always have the clear value set to
false. Previously the initial ttm_tt would be created in
ttm_bo_validate() with the clear parameter always set to true.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Christian König <ckoenig.leichtzumerken@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230130101230.25347-3-matthew.auld@intel.com
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Christian König <christian.koenig@amd.com>