ACPI: video: check for error while searching for backlight device parent

In the Linux kernel, the following vulnerability has been resolved:

ACPI: video: check for error while searching for backlight device parent

If acpi_get_parent() called in acpi_video_dev_register_backlight()

fails, for example, because acpi_ut_acquire_mutex() fails inside

acpi_get_parent), this can lead to incorrect (uninitialized)

acpi_parent handle being passed to acpi_get_pci_dev() for detecting

the parent pci device.

Check acpi_get_parent() result and set parent device only in case of success.

Found by Linux Verification Center (linuxtesting.org) with SVACE.