drm/i915: Fix i915_dma_map_single positive error code

i915_dma_map_single relies on dma_mapping_error, which returns positive
error codes. Found by static checker.

Introduced by commit 678d96fb
("drm/i915: Track GEN6 page table usage").

v2: Return negative error code and renamed commit title. (Dan)
v3: Missing reported-by tag (Daniel)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -311,7 +311,10 @@ static inline int i915_dma_map_page_single(struct page *page,
 	struct device *device = &dev->pdev->dev;
 
 	*daddr = dma_map_page(device, page, 0, 4096, PCI_DMA_BIDIRECTIONAL);
-	return dma_mapping_error(device, *daddr);
+	if (dma_mapping_error(device, *daddr))
+		return -ENOMEM;
+
+	return 0;
 }
 
 static void unmap_and_free_pt(struct i915_page_table_entry *pt,