msm: vidc: Amend calculation of buffer sizes in VENUS_BUFFER_SIZE

Add 8KB worth of padding for extradata. This is required to accommodate
some of the larger extradata types that didn't fit into the residual
space between the actual buffer size and its aligned size.

CRs-Fixed: 647378
Change-Id: I550f806079dfbdece229f68ffdfc5c0e20b3c9e1
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>