drm/ttm: Remove redundant code in ttm_tt_init_fields

Remove redundant assignment code for ttm->caching as it's overwritten
just a few lines later.

v2:
 - Update the commit message.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531053051.3453509-1-Jun.Ma2@amd.com