msm: vidc: Ensure validity of shared Q indices

Video driver and firmware communicates over shared queue.
The queue header has the indices which synchronizes the read
and write between the driver and firmware modules.
This change ensures that the indices are within the valid
range before accessing them.

CRs-fixed: 2345481
Change-Id: I8da6bb4218a5b8ec0e2e2c7b87f6cc9eec21bd16
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>