drm/i915/gvt: Make mmio_attribute as type u8 to save 1.5MB memory

Type u8 is big enough to contain all MMIO attribute flags. As the
total MMIO size is 2MB so we saved 1.5MB memory.

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