hw/display: factor out the scanout blob to fb conversion

hw/display: factor out the scanout blob to fb conversion

There are two identical sequences of a code doing the same thing that
raise warnings with Coverity. Before fixing those issues lets factor
out the common code into a helper function we can share.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Message-ID: <20241111230040.68470-2-alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>