media: hantro: Prevent encoders from using post-processing

The post-processing feature is meant to be used by decoding
only. Prevent encoding jobs from enabling it.

Fixes: 8c2d66b036c77 ("media: hantro: Support color conversion via post-processing")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
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