vulkan: return VK_NOT_READY when no queries are available

Fixes a validation issue.
The issue is that the function gets called before we've sumitted a frame
for decoding to that context. However, we cannot run queries before
they've been reset, which happens at submission time.
As we'd need to otherwise run a command queue at init-time, just check
if submissions have happened.

libavutil/vulkan.h

@@ -154,6 +154,7 @@ typedef struct FFVkExecContext {
     uint32_t idx;
     const struct FFVkExecPool *parent;
     pthread_mutex_t lock;
+    int had_submission;
 
     /* Queue for the execution context */
     VkQueue queue;