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.

@He_Junyan: I don't have the hardware to test it, but what my compiler warns about.