drm/i915/gvt: Cleanup struct intel_gvt_mmio_info

The size, length, addr_mask fields actually are not necessary. Every
tracked mmio has DWORD size, and addr_mask is a legacy field.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>