drm/ttm: Fix a regression causing kernel oops'es

The branch is explicitly taken if ttm == NULL which means that to avoid
a null pointer reference the ttm object can not be used inside. Switch
back to dst_mem to avoid kernel oops'es.

This fixes kernel oops'es with any buffer objects which don't have ttm_tt,
e.g. with vram based screen objects on vmwgfx.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Fixes: e3c92eb4

 ("drm/ttm: rework on ttm_resource to use size_t type")
Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111175015.1134923-1-zack@kde.org
Signed-off-by: Christian König <christian.koenig@amd.com>