msm: vidc: use actual bitrate for decoder bw voting

Use the max filled_len in pending etbs to calculate
the actual bitrate for decoder bw voting, instead of
using pre-defined values in lookup table.
Refine bitrate calculation in low bitrate case.

Change-Id: I0624fc014273efcc0d15c480e731b4aba1b4b941
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>
Signed-off-by: Karthikeyan Periasamy <kperiasa@codeaurora.org>