drm/msm/mdss: Handle the reg bus ICC path

Apart from the already handled data bus (MAS_MDP_Pn<->DDR), there's
another path that needs to be handled to ensure MDSS functions properly,
namely the "reg bus", a.k.a the CPU-MDSS interconnect.

Gating that path may have a variety of effects, from none to otherwise
inexplicable DSI timeouts.

Provide a way for MDSS driver to vote on this bus.

A note regarding vote values. Newer platforms have corresponding
bandwidth values in the vendor DT files. For the older platforms there
was a static vote in the mdss_mdp and rotator drivers. I choose to be
conservative here and choose this value as a default.

Co-developed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/570164/
Link: https://lore.kernel.org/r/20231202224247.1282567-5-dmitry.baryshkov@linaro.org