drm/i915/gvt: introduce intel_vgpu_reset_cfg_space to reset configuration space

This patch introduces a new function intel_vgpu_reset_cfg_space()
to reset vGPU configuration space. This function will unmap gttmmio
and aperture if they are mapped before. Then entire cfg space will
be restored to default values.

Currently we only do such reset when vGPU is not owned by any VM
so we simply restore entire cfg space to default value, not following
the PCIe FLR spec that some fields should remain unchanged.

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