msm: vidc: Fix mbpf calculation for active session

Fix macro block per frame calculation by not
considering sessions which have state greater
than or equal to MSM_VIDC_STOP_DONE.

Change-Id: Ie25e52e946386fcf226b0f9c073c1b022cf1ac0c
Signed-off-by: Chandrakant I Viraktamath <civirakt@codeaurora.org>