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: Ia7529eb9c254d35ffbfc2dbec4e51e0af2191219
CRs-Fixed: 804130
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>