drm/amdgpu: fix aperture mapping leak

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: fix aperture mapping leak

amdgpu_pci_remove() calls drm_dev_unplug() before invoking the driver
fini routines. This causes drm_dev_enter() in amdgpu_ttm_fini() to
always return false, so iounmap(aper_base_kaddr) never runs on normal
driver unload, leaving an orphaned entry in the x86 PAT interval tree.

On connected_to_cpu hardware, the aperture is mapped write-back (WB) via
ioremap_cache(). On reload, IP discovery calls memremap(..., MEMREMAP_WC)
over the same range. The WC vs WB conflict causes:

ioremap error for 0x..., requested 0x1, got 0x0
amdgpu: discovery failed: -2

Fix by switching to devres-managed mappings so cleanup is guaranteed
regardless of drm_dev_enter() state:

- connected_to_cpu path: devm_memremap(MEMREMAP_WB). For
IORESOURCE_SYSTEM_RAM ranges this takes the try_ram_remap() shortcut,
returning __va(offset) from the existing kernel direct map. No new
ioremap VA or PAT entry is created, so there is nothing to orphan.

- dGPU path: devm_ioremap_wc() registers iounmap() as a devres action,
guaranteeing cleanup at device_del() time.

Also remove iounmap(aper_base_kaddr) from amdgpu_device_unmap_mmio()
since the mapping is now devres-owned.

v2: Remove redundant x86_64 guard (Lijo)

(cherry picked from commit d871e99879cb5fd1fa798b006b4888887e63a17a)