drm/msm/a6xx: Store primFifoThreshold in struct a6xx_info

The if-else monster is so unmaintainable that one case is repeated
twice. Get rid of it.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/611092/


[add missing entry to a615 catalog to resolve conflict]
Signed-off-by: Rob Clark <robdclark@chromium.org>