pvr: Only destroy the physcial device if it exists

Without this, it segfaults on instance destruction which brings down the
Vulkan app if you have the driver installed on your system, even if you
have no PowerVR hardware.

Reviewed-by: Rajnesh Kanwal <rajnesh.kanwal@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18330>

src/imagination/vulkan/pvr_device.c

@@ -240,7 +240,8 @@ void pvr_DestroyInstance(VkInstance _instance,
    if (!instance)
       return;
 
-   pvr_physical_device_finish(&instance->physical_device);
+   if (instance->physical_devices_count > 0)
+      pvr_physical_device_finish(&instance->physical_device);
 
    VG(VALGRIND_DESTROY_MEMPOOL(instance));