drm/msm: Only enable A6xx LLCC code on A6xx

Using this code on A5xx (and probably older too) causes a
smmu bug.

Fixes: 474dadb8

 ("drm/msm/a6xx: Add support for using system cache(LLC)")
Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>