drm/vmwgfx: Fix handling of errors returned by 'vmw_cotable_alloc()'

'vmw_cotable_alloc()' returns an error pointer on error, not NULL.
Propagate the error code, instead of returning -ENOMEM unconditionally
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_context.c

@@ -209,8 +209,8 @@ static int vmw_gb_context_init(struct vmw_private *dev_priv,
 		for (i = 0; i < SVGA_COTABLE_DX10_MAX; ++i) {
 			uctx->cotables[i] = vmw_cotable_alloc(dev_priv,
 							      &uctx->res, i);
-			if (unlikely(uctx->cotables[i] == NULL)) {
-				ret = -ENOMEM;
+			if (unlikely(IS_ERR(uctx->cotables[i]))) {
+				ret = PTR_ERR(uctx->cotables[i]);
 				goto out_cotables;
 			}
 		}