ACPI / OSI: Add OEM _OSI string to enable dGPU direct output

For HP Inc. mobile workstation with hybrid graphics support, dGPU can
directly output to external monitors; however, Nvidia and AMD's Linux
drivers aren't able to support this feature.

The OEM _OSI string "Linux-HPI-Hybrid-Graphics" is used by BIOS to
implement dGPU direct output to external monitors.

The form of the OEM _OSI strings is defined by each OEMs and is
discussed in Documentation/acpi/osi.txt.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>