arm64: dma-mapping: Simplify pgprot handling

Since __get_dma_pgprot() does The Right Thing(TM) in the non-coherent
case, and the non-cacheable alias for DMA buffers is private to the
kernel anyway, we can simplify things slightly and make the code more
readable by just using PAGE_KERNEL as the base pgprot.
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/dma-mapping.c

@@ -144,6 +144,7 @@ static void *__dma_alloc(struct device *dev, size_t size,
 	struct page *page;
 	void *ptr, *coherent_ptr;
 	bool coherent = is_device_dma_coherent(dev);
+	pgprot_t prot = __get_dma_pgprot(attrs, PAGE_KERNEL, false);
 
 	size = PAGE_ALIGN(size);
 
@@ -171,9 +172,7 @@ static void *__dma_alloc(struct device *dev, size_t size,
 	/* create a coherent mapping */
 	page = virt_to_page(ptr);
 	coherent_ptr = dma_common_contiguous_remap(page, size, VM_USERMAP,
-				__get_dma_pgprot(attrs,
-					__pgprot(PROT_NORMAL_NC), false),
-					NULL);
+						   prot, NULL);
 	if (!coherent_ptr)
 		goto no_map;