ASoC: msm: qdsp6v2: check if backend active before querying RMS

RMS query to dsp should be done only when the relevant back end is
active to avoid un-necessary errors. Put a check to see if the
backend is active.

CRs-Fixed: 597886
Change-Id: I1ed402272eb34aa6120adbb97d1627a3f7f9b3c5
Signed-off-by: Jitendra Singh Naruka <jnaruk@codeaurora.org>