msm: vidc: Fix incorrect fbd count issue

Currently fbd count will increase not only when firmware
report fbd, but also when some pending ftbs hold by driver
are flushed out. This causes fbd count larger than
actual value after seek/flush events, and will cause
DCVS got wrong fw_pending_bufs value and set lower
clock rate than expected.
Modify the place for fbd count++, only do this when handle
firmware reported fbds.

CRs-Fixed: 2086192
Change-Id: I9989fcf86d51873d99a51adf18079353070a2a3f
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>