msm: vidc: Count an open instance as active for DCVS

If the driver waits until reaching the START_DONE state,
it would be too late to inform the firmware about the extra
dcvs buffer count, which gets added if the session is
deemed eligible for DCVS. Without this change, the actual
buffer count set on firmware does not include the additional
dcvs buffer count. This causes an incorrect calculation of the
low dcvs threshold value, resulting in frame drops.

Change-Id: I337078ad501c2f1e4a3c5ca13a35fced6fb2100c
Signed-off-by: Arun Menon <avmenon@codeaurora.org>