dsp: Do not send ABR parameters if bitrate is fixed

ABR encoder config parameters should be sent to DSP
only when ABR is enabled. Do not send these parameters
for fixed bitrate.

Change-Id: Icf7e4feef65d01c8ae3f6c6123d490cadef46c97
Signed-off-by: Aniket Kumar Lata <alata@codeaurora.org>