ACPI / video: remove unnecessary type casting

Remove type casting from assignments involving void pointers
in two places in drivers/acpi/video.c.

[rjw: Changelog]
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>