msm: vdec: Updating DCVS buf count if thumbnail mode is enabled

Initially for the Decode/Encode session DCVS count upadated with
four. Now from the app thumbnail mode is enabled and the DCVS count
is not updated to zero, immediately app has queried req_buf, here
the driver setting actual count to five and fw returned eight it
in turn returned to app, the count is not equal to one with this
app is exiting with error

CRs-Fixed: 2047275
Change-Id: I001229595a4b01ee5a70f18f553972aa5b737c4f
Signed-off-by: Deepak Kushwah <dkushwah@codeaurora.org>