virgl: wait after a flush

virgl: wait after a flush

We really need to wait under certain circumstances, or we can end
up writing to memory the same time the host is reading.

Partial revert of d6dc68 ("virgl: use uint16_t mask instead of separate booleans").

Test cases:
   - dEQP-GLES31.functional.texture.texture_buffer.render_modify.as_vertex_array.bufferdata
     on vtest protocol version 2
   - Flickering during Alien Isolation
Fixes: d6dc68 ("virgl: use uint16_t mask instead of separate booleans")
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-By: Piotr Rak <p.rak@samsung.com>