msm: camera: fd: Fix race condition in resetting ready_to_process

Do not unlock frame_req_mutex until the request is added
into processing list after submitting the frame to hw.
This can cause a race condition where Flush comes before
adding the frame into processing list and thinks there
is no frame with HW. And there is a possibility that
FrameDone IRQ thread can get the mutex and thinks no frame
in processing list - causing never to reset the flag
ready_to_process. This will cause all subsequent camera
usage to fail as requests will never be submitted to hw.

CRs-Fixed: 2600443
Change-Id: Icd12e155192fccd9adbc7b774b073ca623f1d7ba
Signed-off-by: Pavan Kumar Chilamkurthi <pchilamk@codeaurora.org>