drm/amdgpu: fix aperture mapping leak

Summary

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)