drm/i915/gvt: Add helper for tuning MMIO hash table

We count all the tracked virtual MMIO registers, which can help us to
tune the MMIO hash table.

v2: Move num_tracked_mmio into gvt structure.

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