Workaround broken PCIe speeds on Intel Arc

[Steve-Tech]

Hi again,

Intel Arc Alchemist has a weird (hardware?) bug, where the card and one of it's two PCIe bridges report their link speed as PCIe 1.0 x1, this PR implements a workaround to get the correct speed from it's first PCIe bridge.

e.g. lspci -tv:

-[0000:00]-+-01.1-[01-04]----00.0-[02-04]--+-01.0-[03]----00.0  Intel Corporation DG2 [Arc A770]
                                           \-04.0-[04]----00.0  Intel Corporation DG2 Audio Controller
             ^ Correct       ^ Incorrect     ^ Incorrect

I have checked a few probes on the Linux Hardware Database to verify whether it's just my system or card, and it's not. There is also a bug report on Intel's website with some info.

I have no idea if this is fixed in Battlemage.

I've also added Intel to the libdrm detection in src/CMakeLists.txt, and moved AMDGPU and MSM support checks out of the libdrm detection block since FATAL_ERROR will cause CMake will exit anyway. I hope this is okay.

Thanks,
Steve

[Syllo]

Sounds reasonable, but I'd rather only enable this for the Alchemist devices (ARC) devices for now.

Looking at Intel's hardware table it seems that all the ARC PCI IDs starts with "0x56" so doing a check like if (dev_id & 0xff00 == 0x5600) before entering your workaround would be better imho.

[Steve-Tech]

Yeah good point, it also looks like the PCI bridges are usually '4fa0' and '4fa4', so I can check those too. I'll definitely implement this sometime after next week!