libs: encoder: refine bitrate control mode
1.Remove useless guard of all bitrate control mode's guard except MB which is define in VA-API version 0.39.1.
2.Add MB ratecontrol mode to get its string.
Edited by wangfei
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1.Remove useless guard of all bitrate control mode's guard except MB which is define in VA-API version 0.39.1.
2.Add MB ratecontrol mode to get its string.