The msmpeg4 decoders/encoders share a common set of prerequisites,
ergo it makes sense to use common subsystems for them. This also
allows to remove the CONFIG_MSMPEG4_DECODER/ENCODER ad-hoc defines
(which violated the CONFIG_ namespace).
Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>