iris: Detect DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT kernel support

We will use this for implementing deferred flushes in the next commit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3802>

src/gallium/drivers/iris/iris_screen.h

@@ -168,6 +168,10 @@ struct iris_screen {
       bool always_flush_cache;
    } driconf;
 
+   /** Does the kernel support various features (KERNEL_HAS_* bitfield)? */
+   unsigned kernel_features;
+#define KERNEL_HAS_WAIT_FOR_SUBMIT (1<<0)
+
    unsigned subslice_total;
 
    uint64_t aperture_bytes;