msm: vidc: optimize encoder ftb cache ops

Store the fbd filled length of each output buffer.
When doing cache invalidate for ftb, use the stored
length in previous fbd of the same buffer, instead of
using the buffer alloc size.

CRs-Fixed: 2406814
Change-Id: I6fd113f8c7d99f5ef885fbf773e427cf48b56095
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>