What is the use of 'i2c_get_clientdata“ and ”i2c_set_clientdata"?

Those functions are used to get/set the void *driver_data pointer that is part of the struct device, itself part of struct i2c_client.

driver_data

Private pointer for driver specific info.

struct i2c_client {
  unsigned short flags;
  unsigned short addr;
  char name[I2C_NAME_SIZE];
  struct i2c_adapter * adapter;
  struct device dev;
  int irq;
  struct list_head detected;
  i2c_slave_cb_t slave_cb;
}; 

struct device {
  struct device * parent;
  struct device_private * p;
  struct kobject kobj;
  const char * init_name;
  const struct device_type * type;
  struct mutex mutex;
  struct bus_type * bus;
  struct device_driver * driver;
  void * platform_data;
  void * driver_data;
  struct dev_pm_info power;
  struct dev_pm_domain * pm_domain;
#ifdef CONFIG_PINCTRL
  struct dev_pin_info * pins;
#endif
#ifdef CONFIG_NUMA
  int numa_node;
#endif
  u64 * dma_mask;
  u64 coherent_dma_mask;
  unsigned long dma_pfn_offset;
  struct device_dma_parameters * dma_parms;
  struct list_head dma_pools;
  struct dma_coherent_mem * dma_mem;
#ifdef CONFIG_DMA_CMA
  struct cma * cma_area;
#endif
  struct dev_archdata archdata;
  struct device_node * of_node;
  struct acpi_dev_node acpi_node;
  dev_t devt;
  u32 id;
  spinlock_t devres_lock;
  struct list_head devres_head;
  struct klist_node knode_class;
  struct class * class;
  const struct attribute_group ** groups;
  void (* release) (struct device *dev);
  struct iommu_group * iommu_group;
  bool offline_disabled:1;
  bool offline:1;
};