msm: vidc: Add memory barrier after queue header update

Add memory barrier after updating queue header variables
to ensure main memory is updated so that video hardware
reads the updated header values.

CRs-Fixed: 2135048
Change-Id: I1a2778bee16c9093284c4d33980e6985c279f499
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>