msm: vidc: Change in input buffer size calculation

Right now, input buffer size is calculated based on
maximum supported height and width returned from FW.
These values are not true representation as they are
calculated for rotation usecase. Driver needs to use
max MB supported from FW. This change fixes the same.

CRs-Fixed: 599818
Change-Id: I5b5f7d0db1088a4bc16ec7a32b31e1f763d5da7c
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>