backlight: tps65217_bl: Add missing platform_set_drvdata in tps65217_bl_probe

Otherwise, we got NULL derefernce while calling backlight_device_unregister()
in tps65217_bl_remove().

Also convert to use module_platform_driver.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>