i915/gvt: seperate tracked MMIO table from handlers.c

To support the new mdev interfaces and the re-factor patches from
Christoph, which moves the GVT-g code into a dedicated module, the GVT-g
MMIO snapshot still needs to be saved in i915 so that the inital clean HW
state can be used for the further vGPU. Seperate the tracked MMIO table
from GVT-g, so that GVT-g and i915 can both use it.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Zhi Wang <zhi.wang.linux@gmail.com>