Change in coreboot[master]: soc/intel/graphics: Add override api for graphics_get_memory_base()

[Wonkyu Kim (Code Review)]

Attention is currently required from: Subrata Banik, Nico Huber, Ethan Tsao, Ravishankar Sarawadi, Tim Wawrzynczak, Paul Menzel, Raj Astekar, Patrick Rudolph.
Wonkyu Kim has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/61389 )

Change subject: soc/intel/graphics: Add override api for graphics_get_memory_base()
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Patch Set 8:

(1 comment)

Patchset:

PS7:
> So is it an offset in the PCI BAR? or completely differently configured? It […]

PCI config space offset 0x18 returns memory bar address and display memory(memory_base) which we need to be returned in the fucntion(graphics_get_memory_base) has same address as graphic IP maps.
That what we have in graphics_get_memory_base implmentation.

In future platform, PCI config 0x18 still has memory bar but display memory may not same as memory bar address.
So, we need to overide address based on graphic IP address mapping per platform.