radeonsi/vcn: Don't reinitialize encode session on bitrate/fps change

When bitrate or fps change is detected, only update rate control
parameters instead of completely reinitializing encode session.

This fixes an issue where if application changed bitrate or fps often,
the output bitrate would significantly overshoot the target bitrate in some
cases. In other cases, the output bitrate would be extremely low instead.

Cc: mesa-stable

Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <mesa/mesa!27548>