auxdisplay: linedisp: encapsulate container_of usage within to_linedisp

auxdisplay: linedisp: encapsulate container_of usage within to_linedisp

Replace direct container_of() calls with a to_linedisp() helper function
throughout the line-display auxdisplay library module. This abstraction
prepares for upcoming dual-mode support where linedisp context retrieval
will need to handle both dedicated child devices and attached parent
auxdisplay devices.

No functional changes in this patch.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>