msm: vidc: Update bitstream buffer size for secure case

Existing logic to calculate buffer size is based on
the max bitrate supported during a secure session. Even
the peak bitrate calculation is not sufficient to arrive
at the required frame size.
Going back to earlier logic based on macroblocks per frame.

CRs-Fixed: 2467242
Change-Id: I0fe1768036f19ad2f1d08222e3ed239be7a2e27c
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>