drm/qxl: track primary bo

Track which bo is used as primary surface.  With that in place we don't
need the primary_created flag any more, we can just check the primary bo
pointer instead.

Also verify we don't already have a primary surface in
qxl_io_create_primary().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-13-kraxel@redhat.com