vulkan-device-select: Don't leak drmDevicePtr

ASAN found a leak:

```
Direct leak of 1440 byte(s) in 10 object(s) allocated from:
    #0 0x4a9a92 in calloc (build-Monado-CMake/src/xrt/targets/service/monado-service+0x4a9a92)
    #1 0x7fdf82afed06 in drmDeviceAlloc build-drm/../drm/xf86drm.c:3933:14
    #2 0x7fdf82b00203 in drmProcessPciDevice build-drm/../drm/xf86drm.c:3965:11
    #3 0x7fdf82b00203 in process_device build-drm/../drm/xf86drm.c:4359:16
    #4 0x7fdf82b0485e in drmGetDevice2 build-drm/../drm/xf86drm.c:4528:15
    #5 0x7fdf70751113 in device_select_find_xcb_pci_default ../src/vulkan/device-select-layer/device_select_x11.c:95:13
    #6 0x7fdf70751113 in get_default_device ../src/vulkan/device-select-layer/device_select_layer.c:395:21
    #7 0x7fdf70751113 in device_select_EnumeratePhysicalDevices ../src/vulkan/device-select-layer/device_select_layer.c:456:33
```

Cc: mesa-stable
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14068>

src/vulkan/device-select-layer/device_select_x11.c

@@ -67,6 +67,8 @@ int device_select_find_xcb_pci_default(struct device_pci_info *devices, uint32_t
   int scrn;
   xcb_connection_t *conn;
   int default_idx = -1;
+  drmDevicePtr xdev = NULL;
+
   conn = xcb_connect(NULL, &scrn);
   if (!conn)
     return -1;
@@ -91,7 +93,6 @@ int device_select_find_xcb_pci_default(struct device_pci_info *devices, uint32_t
   if (dri3_fd == -1)
     goto out;
 
-  drmDevicePtr xdev;
   int ret = drmGetDevice2(dri3_fd, 0, &xdev);
   close(dri3_fd);
   if (ret < 0)
@@ -113,7 +114,9 @@ int device_select_find_xcb_pci_default(struct device_pci_info *devices, uint32_t
     if (default_idx != -1)
       break;
   }
+
 out:
+  drmFreeDevice(&xdev); /* Is NULL pointer safe. */
   xcb_disconnect(conn);
   return default_idx;
 }