libhevcenc: Release async control command buffer

Async command control buffer is not being released after use.
This is corrected.
Also ensured dynamic bitrate doesn't go below min supported bitrate

Test: hevcenc -c vid_enc_cfg.txt
Bug: 147574800
Change-Id: I0e31092bb9c51ce5d22798aab6004d1654ff985e
3 files changed