drm/amdgpu: add coreboot workaround for KV/KB

Coreboot seems to have a problem correctly setting up access to the stolen VRAM
on KV/KB. Use the direct access only when necessary.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-and-tested-by: Fredrik Bruhn <fredrik.bruhn@unibap.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c

@@ -381,7 +381,8 @@ static int gmc_v7_0_mc_init(struct amdgpu_device *adev)
 	adev->gmc.aper_size = pci_resource_len(adev->pdev, 0);
 
 #ifdef CONFIG_X86_64
-	if (adev->flags & AMD_IS_APU) {
+	if (adev->flags & AMD_IS_APU &&
+	    adev->gmc.real_vram_size > adev->gmc.aper_size) {
 		adev->gmc.aper_base = ((u64)RREG32(mmMC_VM_FB_OFFSET)) << 22;
 		adev->gmc.aper_size = adev->gmc.real_vram_size;
 	}