drm/msm/a6xx: Use barriers while updating HFI Q headers

drm/msm/a6xx: Use barriers while updating HFI Q headers

To avoid harmful compiler optimizations and IO reordering in the HW, use
barriers and READ/WRITE_ONCE helpers as necessary while accessing the HFI
queue index variables.

Fixes: 4b565ca5a2cb ("drm/msm: Add A6XX device support")
Signed-off-by: Akhil P Oommen <akhilpo@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/714653/
Message-ID: <20260327-a8xx-gpu-batch2-v2-1-2b53c38d2101@oss.qualcomm.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>