media: rkvdec: Initialize the m2m context before the controls

Setting up the control handler calls into .s_ctrl ops. While validating
the controls the ops may need to access some of the context state, which
could lead to a crash if not properly initialized.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>