drm/msm/a5xx: Disable flat shading optimization

Port over the command from downstream to prevent undefined
behaviour.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>