drm/nouveau: fix build failures on all non ARM.

gk20a is an ARM only GPU, so we can just do the correct thing on
ARM but fail on other architectures. The other option was to use
SWIOTLB as the define, which means phys_to_page exists, but
this seems clearer.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c

@@ -133,18 +133,24 @@ gk20a_instobj_size(struct nvkm_memory *memory)
 static void __iomem *
 gk20a_instobj_cpu_map_dma(struct nvkm_memory *memory)
 {
+#if defined(CONFIG_ARM) || defined(CONFIG_ARM64)
 	struct gk20a_instobj_dma *node = gk20a_instobj_dma(memory);
 	struct device *dev = node->base.imem->base.subdev.device->dev;
 	int npages = nvkm_memory_size(memory) >> 12;
 	struct page *pages[npages];
 	int i;
 
+	/* we shouldn't see a gk20a on anything but arm/arm64 anyways */
 	/* phys_to_page does not exist on all platforms... */
 	pages[0] = pfn_to_page(dma_to_phys(dev, node->handle) >> PAGE_SHIFT);
 	for (i = 1; i < npages; i++)
 		pages[i] = pages[0] + i;
 
 	return vmap(pages, npages, VM_MAP, pgprot_writecombine(PAGE_KERNEL));
+#else
+	BUG();
+	return NULL;
+#endif
 }
 
 static void __iomem *