drm/i915/gvt: ignore unexpected pvinfo write

There is pvinfo writing come from vgpu might be unexpected, like
writing to one unknown address, GVT-g should do as reserved register
to discard any invalid write. Now GVT-g lets it write to the vreg
without prompt error message, should ignore the unexpected pvinfo
write access and leave the vreg as the default value.

For possible guest query GVT-g host feature, this returned proper
value instead of wrong guest setting.

v2: ignore unexpected pvinfo write instead of return predefined value

Fixes: e39c5add ("drm/i915/gvt: vGPU MMIO virtualization")
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>