ui: avoid unnecessary memory operations in vnc_refresh_server_surface()

Check the dirty bits in advance to avoid unnecessary memory operations.
In the case that guest surface has different format than the server,
but it does not have dirty bits which means no refresh is actually needed,
the memory operations is not necessary.

Signed-off-by: Jianxian Wen <jianxian.wen@verisilicon.com>
Signed-off-by: Lu Gao <lu.gao@verisilicon.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <4C23C17B8E87E74E906A25A3254A03F4FA22100C@SHASXM06.verisilicon.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>