msm: vidc: Disable DCVS completely if disabled from debugfs

If decoder DCVS was disabled through debugfs, the driver was still
checking if DCVS was supported for the session and enable or
disable the inst->dcvs_mode flag accordingly. Since bitrate scaling
is dependent on DCVS being turned off, enabling the dcvs mode flag
would disrupt bitrate clock scaling.

Change-Id: I1ad6d47bfcac5cec97b74848b87a107cd7286443
Signed-off-by: Arun Menon <avmenon@codeaurora.org>