drm/i915/gvt: Check if get_next_pt_type() always returns a valid value

According to gtt_type_table[] function get_next_pt_type() may returns
GTT_TYPE_INVALID in some cases. To prevent driver to try to create memory
page with invalid data type, additional check is added.

Signed-off-by: Aleksei Gimbitskii <aleksei.gimbitskii@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Colin Xu <colin.xu@intel.com>
Reviewed-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>