drm/vmwgfx: stop setting multiple domain flags

Setting more than one placement flag is illegal.
Fix this by using the current flags.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/389824/?series=81551&rev=1