drm/gem-vram: handle NULL bo->resource in move callback

The ttm BO now initially has NULL bo->resource, and leaves the driver
the handle that. However it looks like we forgot to handle that for
ttm_bo_move_memcpy() users, like with vram-gem, since it just silently
returns zero. This seems to then trigger warnings like:

WARNING: CPU: 0 PID: 1 at drivers/gpu/drm/drm_gem_vram_helper.c:255 drm_gem_vram_offset (??:?)

Fix this by calling move_null() if the new resource is TTM_PL_SYSTEM,
otherwise do the multi-hop sequence to ensure can safely call into
ttm_bo_move_memcpy(), since it might also need to clear the memory.
This should give the same behaviour as before.

While we are here let's also treat calling ttm_bo_move_memcpy() with
NULL bo->resource as programmer error, where expectation is that upper
layers should now handle it.

Fixes: 18025378 ("drm/ttm: stop allocating dummy resources during BO creation")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230208145319.397235-1-matthew.auld@intel.com


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>