msm: camera: jpeg: Add mutex lock to protect jpeg list corruption

Due to race conditions, situation can arise where the process_irq
and flush for jpeg are being handled in parallel. This will cause
the jpeg list corruption.
This commit protects the code of adding back to free list
in process_irq with the mutex.

CRs-Fixed: 2578247
Change-Id: I28ee48bc0d5cfcf3ae4a936b2eb2976226ad88d5
Signed-off-by: Gaurav Jindal <gjindal@codeaurora.org>