va: av1dec: Avoid structure overwrite.

VADecPictureParameterBufferAV1.mode_control_fields.bits were filled
twice, overwriting to zeros the first assignation. This patch unifies
both assignations.

Also it makes explicit an enum casting between libva and gstreamer; it
removes the assignation to zero a deprecated parameter; and use an
appropriate assertion.

Part-of: <!2223>
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