drm/i915/gvt: always use i915_reg_t for MMIO handler definition

Always requires properly defined i915_reg_t type for MMIO handler
definition.

Fix kasan warning of "drivers/gpu/drm/i915/gvt/handlers.c:2397:1: error: the frame size of 32120 bytes is larger than 8192 bytes"
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>