msm: vidc: Fix output buffer count in thumbnail mode

In thumbnail mode when client opens multiple sessions, decoder sets
minimum output buffer count to 6 thus resulting in OOM scenario. This
change updates the output buffer count in case of thumbnail mode and
maintains the min count as 6 for regular decoding

Change-Id: I8b6df51a7a554f65813125aed98208f5e0d04471
Signed-off-by: Ashray Kulkarni <ashrayk@codeaurora.org>
Signed-off-by: Balamurugan Alagarsamy <balaga@codeaurora.org>
Signed-off-by: Bharat Pawar <bpawar@codeaurora.org>