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.