drm/hisilicon/hibmc: Checking for NULL instead of IS_ERR()

The drm_dev_alloc() function returns error pointers.  It never returns
NULLs.

Fixes: 5e0df3a0 ("drm/hisilicon/hibmc: Add hisilicon hibmc drm master driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161124113545.GP17225@mwanda

drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c

@@ -377,9 +377,9 @@ static int hibmc_pci_probe(struct pci_dev *pdev,
 	int ret;
 
 	dev = drm_dev_alloc(&hibmc_driver, &pdev->dev);
-	if (!dev) {
+	if (IS_ERR(dev)) {
 		DRM_ERROR("failed to allocate drm_device\n");
-		return -ENOMEM;
+		return PTR_ERR(dev);
 	}
 
 	dev->pdev = pdev;