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: 65f9f6febf ("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>

drivers/gpu/drm/i915/gvt/handlers.c

@@ -3028,7 +3028,7 @@ int intel_vgpu_mmio_reg_rw(struct intel_vgpu *vgpu, unsigned int offset,
 	gvt_mmio_func func;
 	int ret;
 
-	if (WARN_ON(bytes > 4))
+	if (WARN_ON(bytes > 8))
 		return -EINVAL;
 
 	/*