drm: fix bug introduced in the macro removal

This caused issues with a PCI radeon card.

From: Jon Smirl
Approved-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/drm_scatter.h

@@ -73,7 +73,7 @@ int DRM(sg_alloc)( struct inode *inode, struct file *filp,
 
 	DRM_DEBUG( "%s\n", __FUNCTION__ );
 
-	if (drm_core_check_feature(dev, DRIVER_SG))
+	if (!drm_core_check_feature(dev, DRIVER_SG))
 		return -EINVAL;
 
 	if ( dev->sg )