Fix deqp regression of basic_copyteximage2d*

Disable VREND_STORAGE_GL_IMMUTABLE when feat_egl_image_storage is unavailable

Regression was introduced by
https://gitlab.freedesktop.org/virgl/virglrenderer/merge_requests/269#note_377300

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

src/vrend_renderer.c

@@ -6455,6 +6455,7 @@ static int vrend_renderer_resource_allocate_texture(struct vrend_resource *gr,
           has_feature(feat_egl_image_storage)) {
          glEGLImageTargetTexStorageEXT(gr->target, (GLeglImageOES) image_oes, NULL);
       } else if (has_feature(feat_egl_image_external)) {
+         gr->storage_bits &= ~VREND_STORAGE_GL_IMMUTABLE;
          glEGLImageTargetTexture2DOES(gr->target, (GLeglImageOES) image_oes);
       } else {
          vrend_printf( "missing GL_OES_EGL_image_external extensions\n");