drm/i915/gvt: Change the max length of mmio_reg_rw from 4 to 8

drm/i915/gvt: Change the max length of mmio_reg_rw from 4 to 8

When linux guest access mmio with __raw_i915_read64 or __raw_i915_write64,
its length is 8 bytes.

This fix the linux guest in xengt couldn't boot up as it fail in
reading pv_info->magic.

Fixes: 65f9f6febf12 ("drm/i915/gvt: Optimize MMIO register handling for some large MMIO blocks")
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>