[M] Change in coreboot[main]: drivers/intel/gma/opregion: Use CBFS cache to load VBT

[Jérémy Compostella (Code Review)]

Attention is currently required from: Arthur Heymans, Eran Mitrani, Jakub Czapiga, Julius Werner, Kapil Porwal, Nick Vaccaro, Nico Huber, Tarun.

Jérémy Compostella has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/77886?usp=email )

Change subject: drivers/intel/gma/opregion: Use CBFS cache to load VBT
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Patch Set 33:

(1 comment)

File src/drivers/intel/gma/opregion.c:

https://review.coreboot.org/c/coreboot/+/77886/comment/e1976240_10d04f38 :
PS4, Line 27: cbfs_map
> IIRC, locate_vbt() is called rather early in ramstage. Doesn't this mean […]

According to cscope, `local_vbt()` is called before but also after FSP-S (during GMA initialization).  IMO, considering that CBFS cache is just a scratchpad (not a cache) and that `vbt.bin` is relatively small the only thing that really matters is that we do not call cbfs_map() multiple times. Also to my understanding, `cbfs_unmap()` can only unmap the latest mapped CBFS file.  Therefor, if I add code to unmap it and some other CBFS files got loaded in between then unmap will silently fail.