drm/msm/a6xx: Extend and explain UBWC config

Rename lower_bit to hbb_lo and explain what it signifies.
Add explanations (wherever possible to other tunables).

Port setting min_access_length, ubwc_mode and hbb_hi from downstream.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542764/


Signed-off-by: Rob Clark <robdclark@chromium.org>