drm/i915/gvt: Check if cur_pt_type is valid

Static code analyzer warns that index value for scratch_pt may be equal
to -1. Index value type is intel_gvt_gtt_type_t, so it may be any number
at range -1 to 17. Check first if cur_pt_type and cur_pt_type+1 is valid
values.

v2:
 - Print some error messages if page table type is invalid. (Colin Xu)

v4:
 - Print cur_pt_type in error message. (Colin Xu)

This patch fixed the critial issue #422 reported by klocwork.

Acked-by: Colin Xu <colin.xu@intel.com>
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>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>