msdk: set the right BRCParamMultiplier

BRCParamMultiplier in mfxInfoMFX is a parameter which specifies a
multiplier for bitrate control parameters [1], it impacts TargetKbps,
MaxKbps, BufferSizeInKB and InitialDelayInKB.

[1]: https://software.intel.com/en-us/node/628473
8 jobs for fix-bitrate in 41 minutes and 33 seconds (queued for 1 minute and 6 seconds)