drm/i915/gvt: Rename ggtt related functions to be more specific

Accurate names help to avoid confusing so improve readability.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>