drm/i915/gvt: Remove typedef and let the enumeration starts from zero

Typedef is not recommended in the Linux kernel.The klocwork static code
analyzer takes the enumeration as the full range of intel_gvt_gtt_type_t.
But the intel_gvt_gtt_type_t will never be used in full range. For
example, the GTT_TYPE_INVALID will never be used as an index of an array.
Remove the typedef and let the enumeration starts from zero to pass
klocwork analysis.

This patch fixed the critial issues #483, #551, #665 reported by
klockwork.

v3:
- Remove the typedef and let the enumeration starts from zero.

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>