drm/qxl: drop ttm_mem_reg arg from qxl_hw_surface_alloc()

Not used, is always NULL.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-2-kraxel@redhat.com