msm: vidc: Invalidate encoder output buffer cache

For encode usecase, video driver operates cache always on
the encoded data length rather than the buffer length.
This is an optimization and saves cache operation on unused
memory region.
VB index is used to identify a buffer given to client and fed
again to encoder. Based on that, the filled length of that
buffer is then operated for cache operations.
For certain usecases, it is difficult to identify the buffer
based on VB index. Hence keep a counter to track the max filled
length during the usecase and operate cache on that length

Change-Id: I62cfe876077786ec9d14f541a0080cd2fc124b01
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>