drm/qxl: Fix randbuild error

If DEM_QXL is y and DRM_TTM_HELPER is m, building fails:

drivers/gpu/drm/qxl/qxl_object.o: undefined reference to `drm_gem_ttm_print_info'

Select DRM_TTM_HELPER to fix this.

Fixes: 78d54f1f

 ("drm/qxl: use drm_gem_ttm_print_info")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191008024054.32368-1-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>