msm: camera: ope: Avoid submitting NULL request to CDM

In corner case, there is a chance that userspace can submit
request while flush is ongoing. In some cases submitted request
can get flushed and corresponding data structures  memory gets
freed.

Add a logic to check for pending request before submitting request
to cdm hw.

CRs-Fixed: 2815901
Change-Id: I74096201e204f204c4f15d14698b4e9af6435f55
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>