msm: vidc: Consolidate internal buffers allocation and reuse code

Both Scratch and Persist buffers are internal buffers and allocated
inside driver in a similar way. Consolidating these buffer handling
removes duplication of code.
This also fixes the unnecessary persist buffers warning.

Change-Id: I8e8e4a43b254566c7ceb09844cf8303d4cf6d387
CRs-Fixed: 804130
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>
Signed-off-by: Sowmya Pandiri <spandiri@codeaurora.org>