mesa: flush and wait after creating a fallback texture

Fixes non-deterministic failures in
dEQP-EGL.functional.sharing.gles2.multithread.simple_egl_sync.images.texture_source.teximage2d_render
and others in dEQP-EGL.functional.sharing.gles2.multithread.*

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
diff --git a/src/mesa/main/texobj.c b/src/mesa/main/texobj.c
index 71c6f81..02c4767 100644
--- a/src/mesa/main/texobj.c
+++ b/src/mesa/main/texobj.c
@@ -1051,6 +1051,11 @@
       assert(texObj->_MipmapComplete);
 
       ctx->Shared->FallbackTex[tex] = texObj;
+
+      /* Complete the driver's operation in case another context will also
+       * use the same fallback texture. */
+      if (ctx->Driver.Finish)
+         ctx->Driver.Finish(ctx);
    }
    return ctx->Shared->FallbackTex[tex];
 }