drivers/gop: Resolve GOP child controller ownership

[SnowCheetos]

Summary

• retain device-path resolution as the first GOP-to-PCI ownership proof
• fall back to the explicit UEFI EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER relationship when firmware installs GOP on an unresolvable child handle
• continue reporting unknown rather than guessing from framebuffer address or display class

Why

A Lunar Lake boot using the first framebuffer-source extension enumerated the exact 8086:64a0 display identity, but the source remained unknown and Kore correctly stopped at ownership note 6. On this firmware, the GOP handle does not appear to expose a device path that LocateDevicePath(EFI_PCI_IO_PROTOCOL) can resolve.

The fallback enumerates PCI I/O handles and accepts a candidate only when OpenProtocolInformation(EFI_PCI_IO_PROTOCOL) contains a BY_CHILD_CONTROLLER record whose controller handle is exactly the GOP handle. It returns unknown when that positive UEFI relationship is absent or ambiguous.

Verification

• x86 UEFI target built with --enable-werror
• full BIOS/UEFI multi-architecture build completed with --enable-all --enable-werror
• git diff --check origin/ft/gpu-handoff...HEAD
• Copilot review: no findings

The repository's “targetting trunk” policy check is expected to fail because this integration PR intentionally targets the requested ft/gpu-handoff branch. Both compilation checks are green.

[Copilot]

Pull request overview

This PR improves how the UEFI GOP driver determines framebuffer ownership by PCI endpoint when the GOP handle’s device path cannot be resolved to an EFI_PCI_IO_PROTOCOL provider, adding a standards-based fallback while still avoiding heuristic guesses.

Changes:

• Refactors PCI framebuffer-source extraction into a reusable helper and returns success/failure to drive fallback behavior.
• Adds a fallback path that uses OpenProtocolInformation() and the EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER relationship to match a GOP child handle to its owning PCI I/O controller.
• Updates extension documentation to describe the two-step (device-path first, child-controller relationship second) ownership proof.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
EXTENSIONS.md	Documents the refined GOP-to-PCI ownership resolution strategy and when “unknown” is reported.
common/drivers/gop.c	Implements the child-controller relationship fallback for GOP-to-PCI ownership and refactors PCI source assignment into a helper.

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