virgl/vtest: execute a transfer_get when flushing the front buffer

virgl/vtest: execute a transfer_get when flushing the front buffer

This just moves everything to a helper function -- "flush_front_buffer"
will be used later.

virgl_vtest_resource_map / virgl_vtest_resource_unmap already take
care to map the display target.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-By: Piotr Rak <p.rak@samsung.com>