drm/i915/kvmgt: Add kvmgt debugfs entry nr_cache_entries under vgpu

Add a new debugfs entry kvmgt_nr_cache_entries under vgpu which shows
the number of entry in dma cache.

$ cat /sys/kernel/debug/gvt/vgpu1/kvmgt_nr_cache_entries
10101

v3: fix compiling error for some configuration. (Xiong Zhang <xiong.y.zhang@intel.com>)
v2: keep debugfs layout flat.

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