drm/ttm: ttm_bo_mem_placement doesn't need ctx parameter.

Removed unused parameter.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201021044031.1752624-3-airlied@gmail.com