drm/i915/gvt: expose vGPU context hw id

This exposes vGPU context hw id in mdev sysfs which is used to
do vGPU based profiling. Retrieved vGPU context hw id can be set
through i915 perf ioctl to set profiling for target vGPU.

Cc: Jiao Pengyuan <pengyuan.jiao@intel.com>
Cc: Niu Bing <bing.niu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>