vulkan: check for extension rather than function pointer

The loader ensures only that functions with tagged supported extensions
exist, rather than ensuring only those with supported extensions are
loaded.
As the init function uses Vulkan functions, whose loading requires them
to have the extension flags set, the extension flags are guaranteed
to also exist at this point.

libavutil/vulkan.c

@@ -165,7 +165,7 @@ int ff_vk_load_props(FFVulkanContext *s)
 
     vk->GetPhysicalDeviceQueueFamilyProperties2(s->hwctx->phys_dev, &s->tot_nb_qfs, s->qf_props);
 
-    if (vk->GetPhysicalDeviceCooperativeMatrixPropertiesKHR) {
+    if (s->extensions & FF_VK_EXT_COOP_MATRIX) {
         vk->GetPhysicalDeviceCooperativeMatrixPropertiesKHR(s->hwctx->phys_dev,
                                                             &s->coop_mat_props_nb, NULL);